Welcome to 2017. Thank goodness Spanish football, still the best in the world, is back.

Here are four clubs with the most serious issues that need to be assessed, addressed and overcome in the coming weeks, if the clubs truly want this to be a happy new year.

How will Real Madrid react when they face adversity?

Too many of us start January with a hangover -- be it physical, mental, spiritual, whatever -- and that must be considered Madrid's biggest test, certainly from here until they do battle next month with Napoli in what promises to be a fascinating Champions League round-of-16 tie.

There's something about traveling to Japan as European champions, competing against the kings of South America and returning either to some well-earned time off or the hurly burly of English festive football that seems to stick in the throat of this continent's biggest clubs.

Madrid have suffered in the past. Of the three previous times they've become world champions after a December decider against South American opposition, just once have they gone on to win the Spanish title that season.

In 1998-99, they beat Vasco da Gama then immediately threw away points to Oviedo, Mallorca, Espanyol and Deportivo La Coruna. Barcelona won the league by 11 points.

Vicente Del Bosque's 2002-03 side was good enough to beat Club Olimpia and win their domestic title, albeit on the last day of the Liga season.

But 12 years later, Madrid went from winning 22 straight matches, including the world title against San Lorenzo, to suffering three defeats in January 2015 and ultimately kissing the title goodbye, despite having had a six-point lead.
Cristiano Ronaldo is congratulated by teammates after scoring for Real Madrid in the Club World Cup final against Kashima Antlers.

The curse of the world title? Perhaps. Of the 10 previous Club World Club Cup winners, only four have gone on to lift their domestic title, and more than one coach, Carlo Ancelotti included, has been sacked or quit in the days, weeks or months after ascending to the world throne.

You'd have to suppose that there's a significant mental/psychological element in play here, and Zidane's team is not unique in having approached their latest Tokyo date on "ramming-speed" form.

By definition, any World Club Cup finalist will already have had a good year, given that winning the Champions League is an obligatory precursor to playing in the tournament.

But travelling to Japan, whether to win or to lose, seems to mark a watershed for many players. Time for a breather, time to pat oneself on the back, time for subconscious complacency or arrogance. Call it what you will.

Ahead of the first leg of their Copa del Rey round-of-16 tie against Sevilla on Wednesday, Madrid are within two matches of equaling Barcelona's all-time record of 39 unbeaten games.

But when defeat does come, there can often be a sharp after-effect. It's natural: Suddenly the breaks go against you, character is tested and the habits, formations and substitutions that had been yielding positive results just don't seem to come off.

Sooner or later, this will happen for Madrid. The combination of becoming world champions, the winter break, the cracking of their shield of invincibility and the helter-skelter nature of what Zidane estimates to be "20 games in 70 days" are going to test Madrid to their limits.

How they cope, what fortitude they show and how many of the second-tier players continue to come to the party will go a long way in deciding whether this becomes a second golden year for Madrid's impressive French manager.

Disaster looms for Valencia

Valencia's situation is critical -- not because fan criticism is turning heated and angry and not because they are losing their status as one of Spain's great clubs. It is critical in terms of the continued well-being of the club.

Tuesday's complete humiliation against Celta Vigo in the Copa del Rey was simply the latest cut but not the deepest.

2016 was the calendar year in which Valencia lost more matches than at any time in their history. The next time they are victorious in La Liga, they'll have as many wins -- four -- as they had coaches last year.

But the threat is much greater than on-pitch embarrassment. Valencia's last financial report made it clear that their planning for the next 10 years is, incredibly, specifically based on playing European football every season except this one, increasing TV revenue, season-ticket sales and commercial revenue. Talk about inflated ambitions.

According to the Deloitte Football Money League, Valencia's commercial ranking has plummeted in recent years. Season-ticket revenue is down, as is revenue from shirt manufacturers. Indeed, the club hasn't even been able to attract a shirt sponsor.

But there's worse. That same financial report notes that selling players at a profit is another fundamental part of the club's aim to reduce debt.

What this means is that the same players who need to bring European football every season until 2025 for Valencia to stay on track economically are also those the club is already committed to moving on.

Valencia are 17th, and relegation would complete a spiral of doom. This is a club that teetered on the brink of extinction until Peter Lim bought it and that still has a new stadium that is just two-thirds built.

Unless Los Che sell and sign brilliantly in this month's transfer window, appoint the right coach and perform exceptionally in order to stay up, they are staring disaster right in the face.

The big questions facing Barcelona

Of course the Spanish champions face the usual tests between now and May. Can they retain the Liga title? Can they resuscitate their Champions League personality after running out of gas last spring against Atletico Madrid?

These are hardy perennials, but there are two extraordinary questions hanging over the Camp Nou club, beyond these on-pitch tests.

The first is renewing Lionel Messi's contract, which is currently set to expire in 2018.

He is in the power seat. Make no mistake about that. The recent rash of renegotiations by Gareth Bale, Cristiano Ronaldo and Neymar, as well as the money being offered by Chinese clubs, means Barcelona's genius is perfectly placed to demand -- and receive -- the biggest pay day in football history.

Can the club afford it? With stadium redevelopment around the corner, Real Madrid on the rise and the Premier League richer than ever, they cannot afford not to.

Barcelona's other issue is much harder to pin down. Luis Enrique is out of contract in June, and there's currently no eagerness on his part to resolve whether he's staying or leaving until April. The key question is whether Barcelona can wait that long.

Luis Enrique is within his rights to want to assess how his players react over the next three months, when much of the groundwork to challenge in La Liga, the Copa del Rey and the Champions League is done.

The Asturian needs to know not just whether he has the energy and commitment for another deal but also whether his squad are still wholly engaged. Familiarity, in sport, does breed contempt.

But for the club, the conundrum is that if they wait to start talking until April and the discussions not only drag but then end badly, then Barca are suddenly looking at replacing him and making decisions on whom to sell and whom to buy and are not in the power seat about which coach they would need to attract and sign.

Perhaps "Lucho" will opt to stay. Problem solved, in that case. But that's far from certain, which leaves the Spanish champions with an unwanted game of nerves. They need to guillotine that by asking him to accelerate his yes/no, stay/go debate.

Sevilla must plan ahead

One of life's great skills is to avoid joining Joni Mitchell in her "Big Yellow Taxi." You remember?

Don't it always seem to go
That you don't know what you've got 'til it's gone.

This is Sevilla's golden era. A footballing paradise. Competitive in the league, richer by the season after three straight Europa League wins and now -- based on last season's figures, which will only have increased -- at least €50 million wealthier for having reached the Champions League Round of 16.

But there's a scenario in which, over the next five months, they could suffer the most serious body blows.

The club's football director, Monchi, recently confirmed to me that he still thinks he needs a break from Sevilla, as well as a change of scenery, to recharge his competitive instincts after 16 brilliant years of work, which have taken a floundering, second-division outfit to the peak of European football and wealth.

If Sevilla grant him permission, he'll leave this summer.

Meanwhile, manager Jorge Sampaoli -- a Monchi appointment -- has been terrific so far. But should Luis Enrique leave Barcelona, Sevilla have already discussed what to do in case their Argentinian coach and his Pep Guardiola-disciple assistant, Juanma Lillo, be invited to take over at the Camp Nou.

Then there's Steven N'Zonzi. He is Sevilla's best player, and they're trying to renew his contract, but his current buyout clause -- €30 million -- makes him affordable, and the queue of suitors, which includes Juventus, Manchester City, Paris Saint-Germain and Arsenal, grows by the week.

Whatever they achieve on the playing fields of Spain and Europe this season, it is contingency planning for life after Monchi and what to do about the Sampaoli and N'Zonzi situations that should most occupy the great minds at Sevilla right now.

Before someone paves paradise and puts up a parking lot.