ESPN FC ranks the top five options in order of suitability for Chelsea.

1) Robert Lewandowski

Once whispers emerged that Lewandowski might be unhappy at Bayern Munich this week, it was no surprise to see Chelsea and Manchester United linked.

Roman Abramovich would be foolish not to at least ask the question. Lewandowski is the best pure No. 9 in the world, capable of scoring 30 goals per season -- the mark he has reached in both of the last two Bundesliga campaigns -- while providing a powerful, dynamic focal point for a world-class attack.

Every aspect of Lewandowski's game suggests he would thrive in the Premier League, and he is the only striker potentially available who would represent a definite and immediate upgrade on Costa. At 28, he has already proven himself in the latter stages of the Champions League, and Chelsea could be confident of seeing at least three more seasons of elite production from him.

2) Romelu Lukaku

Chelsea make fewer egregious errors than most in the transfer market, but the failure to get a buy-back clause inserted in the £28 million deal that took Lukaku to Everton in the summer of 2014 must surely count as one.

Three years on, the Belgian looks ready to lead a team challenging for major trophies and all the signs are that he prefers a return to Stamford Bridge above all other possibilities. If Everton refuse to budge from their reported £100m asking price, however, he may not be the most feasible option.

Lukaku has lit up the Premier League in four seasons at Everton, steadily increasing his goal return while improving drastically in other aspects of his game -- most notably his link-up play and heading.

The questions that remain regarding his work ethic are ones that Antonio Conte would back himself to resolve, and it has always seemed harsh to criticise his lack of impact against top opponents in a limited Everton team. Surrounded by Chelsea's greater attacking talents, he would be frightening.

3) Alvaro Morata

Conte has been an avid admirer of Morata for several years. He drove Juventus to sign the Spanish international from Real Madrid before resigning in the summer of 2014, and called him last year to gauge his interest in joining Chelsea.

"I feel indebted to him because he's the coach that most trusted in me, most wanted me, who made me feel I could perform at the highest level," Morata said of Conte in a revealing interview with the Guardian in April. "And yet I've never had the fortune to actually work with him. I'm sure sooner or later I will."

Whether it will be this summer is another question. Lukaku is above him on Chelsea's list, and United's interest in Morata has further complicated things.

There is no doubt that Conte would put him in position to thrive at Chelsea, but Morata cannot boast the same level of scoring pedigree as Lewandowski or even Lukaku. He has reached double figures for league goals just once in his career, scoring 15 in a more peripheral role for Madrid last season.

4) Pierre-Emerick Aubameyang

The reason why Lewandowski's 30 goals were not enough to secure the Bundesliga golden boot last season, Aubameyang is reportedly looking for a change of scenery after four years at Borussia Dortmund.

His lightning speed and talent for attacking space would fit well with a Chelsea team that has proved lethal in transition, while his time at Dortmund has given Aubameyang an excellent grounding in the slick one-touch interchanges that are another attacking feature of Conte's 3-4-3 system.

Dortmund's price -- reportedly in the region of £61m -- is also lower than what Everton or Bayern would likely demand for Lukaku or Lewandowski, though Chelsea are resigned to digging deep into the £60m received for Oscar in January regardless of who they buy.

Replacing the unsettled Costa with Aubameyang would mean tweaking the structure of Chelsea's attack, given that he does not fit the same physical profile as the club's other striker targets. There are also questions over how well the Gabon international's game will age once he enters his 30s.

5) Andrea Belotti

A year ago, Torino's prized asset would not have even been in this conversation. Belotti only emerged as an elite goalscorer -- and a genuine No. 9 -- last season, netting 26 goals in 35 Serie A appearances to finish third behind Edin Dzeko and Dries Mertens in the division's scoring chart.

While being almost exclusively a penalty area finisher like Costa, Belotti can breach defences in different ways; 11 of his Serie A goals last season were scored with his right foot, five with his left and 10 with his head, despite him standing at just 5'10".

Belotti's drive and determination within his stocky, powerful frame appears a good blend to do well in the Premier League. A buyout clause of £86m also makes him a cheaper option than the worst-case scenarios that Chelsea could encounter with other targets.

With just one season of spectacular scoring to his name, however, there's no reason why Belotti should be Chelsea's first port of call.