Manchester United rounded off their summer tour of the USA with their first loss of the pre-season, a 1-0 reverse to Barcelona and their extra-terrestrial trident of Lionel Messi, Luis Suarez and Neymar.

It was the Brazilian's goal just before halftime, after trampling all over a disoriented Antonio Valencia, which ultimately settled the game, but this was a minor blip in an otherwise very successful trip for Jose Mourinho's side.

Here are five things the Red Devils will take away from this tour...

Romelu Lukaku has already shown his quality

Carrying a $75million price tag must be quite the load for a player to bear, even in these days of over-inflated markets.

It's even more weighty for strikers, who must be under immediate pressure to justify the spend in as little time as possible.

Lukaku found that out within one match of suiting up when his turn and miss against LA Galaxy became an instant source of social media memes.

So it must have come as a relief when he opened his account in the very next game, and what a goal it was -- showing all of his assets and most famous liability, that notorious first touch.

His control from Henrikh Mkhitaryan's delightful low cross almost took the ball -- and chance -- away from him, but he recovered, shifted away from the goalkeeper and two defenders, and tucked home. Duck broken.

Throughout the tour, Lukaku showed that he is the striker United needed.

His strength, pace and potential for deadly finishing has seen him double-teamed, opening up opportunities for other players. As the tour progressed, the much-maligned first touch seemed to recede further and further.

What is clear is that as long as Lukaku is firing, United can transform the draws that afflicted them last season into victories.

A new defensive midfielder is a priority

I love Marouane Fellaini. I really do.

He is the plebeian in us all -- a man who does the grafting for his more shiny-edged teammates.

He's good for plugging holes, for making life difficult for the opposition, and for providing air-based artillery when required.

However, as much as I love him, he is not an elite player.

At no time during this five-game series was that more evident than against Barcelona, when he shared the same space with three of the world's best players.

It was quite easy to imagine Paul Pogba in a Barca shirt and absolutely excelling. In the event, he excelled in spite of the reduced talent around him.

In fact, he was the only Man United outfield player who would have looked comfortable strolling into that Barca starting XI. Pogba can do this in the Premier League, too, but only if he has the protection behind that allows him to venture high and free.

At the moment, United do not have it, which is why Mourinho is crying out for at least one new signing. That new signing has to be a holding midfielder. Not just any stop-gap, but an elite player. It may be easier said than done, but acquire such a talent and watch Pogba blossom.

United defending has much improved

Mourinho, not one to bite his tongue, lashed out at his defence after the Barcelona game...perhaps a little unfairly.

They did give away opportunities, but against that attacking threesome, which team doesn't?

Over five games, what has been clear is that the United manager is focused on making his defensive unit greater than the sum of its parts. Bar individual errors, it appears to be working.

Their movement and positioning without the ball, how they hold their lines, keep their shape is all becoming more and more disciplined.

Perhaps the loss of Eric Bailly for two European games is a factor on Mourinho's mind, but he is certainly willing to play the unit over the specific individual...and United are the stronger for it.

Anthony Martial must stay

One bit of skill does not a player make, but looking at Martial owning the Real Madrid defence as he set up Jesse Lingard must make anyone who thought about cutting him loose seriously consider the merits of a psych evaluation.

Young, direct and skilled, Martial represents the very essence of what a Manchester United player should be.

In fact, the best tribute to that piece of magic came via a Twitter post from George Best's widow, who obliquely compared him to the late legend.

There's a very apt Nigerian saying that fits this situation: "What you're looking for in Sokoto (the city) is inside your sokoto (your pants)."

Under no circumstances should United consider trading Martial. What he needs is the self-confidence and the trust of his manager.

It may not come off half the time, but when it does, well, we will be blessed with enchanting moments like those.

United have no concerns between the sticks

How Mourinho will keep two top-class goalkeepers happy is only possible because Sergio Romero obviously knows and accepts the hierarchy.

However, an injury or suspension to David de Gea should leave Mourinho with little cause for worry.

With the club competing in the Champions League this season, it is doubtful if he will continue his policy of playing De Gea in domestic competition and Romero in Europe.

Whatever he decides, he still has two world-class goalkeepers to pick from, and this summer's tour provided ample evidence.