The summer transfer window could be very different in 2021. 

Clubs across Europe have been hit by the COVID-19 pandemic and are facing up to the prospect of an entire season being played behind closed doors. 

With that in mind, even some of Europe's traditional financial powerhouses are going to be in the market for bargains. 

Lionel Messi will be the biggest coup of all.

The Argentine's extortionate wages will be a stumbling block, but he remains determined to leave Barcelona following his dispute with the club this summer. 

However, the Argentine isn't the only big name out of contract who'll be available on a free transfer. 

Marca have compiled a list of 21 of the most high-profile players who are out of contract at the end of the 2020/21 season and there are some incredible signings to be made. 

21 - Mesut Ozil

OK, so perhaps not incredible. Decent, if you will. On his day, at least. Ozil's unhappy situation at Arsenal has seen him left out of their Premier League and Europa League squads altogether. The playmaker still possesses world-class vision and he will be looking to prove a point when he leaves north London. 

p1emd3koa61fa01fcqi561cm011lr9.jpg

20 - Diego Costa 

Costa hasn't been the same player since returning to Atletico Madrid, with 18 goals in 78 appearances. 

19 - Riqui Puig

Ronald Koeman warned Puig he wouldn't get many games this season and the Dutch boss doesn't seem convinced by the youngster yet. That said, he undoubtedly plays the Barcelona way. 

18 - Edinson Cavani

Cavani is only ranked so low because we suspect - and this could come back to haunt us - the days of him scoring almost a half century of goals in a season are long gone. The Uruguayan is yet to score for Manchester United and only hit seven for PSG in Ligue 1 last term. 

17 - Zlatan Ibrahimovic

Age is just a number for Zlatan. He's now 39, which will deter some clubs - and that's why he's so far down the list here - but he's got seven goals in his four Serie A games so far this season. 

p1emd3m0f8rdn1c2ta3d2ic1jnsb.jpg

16 - Eric Garcia

The Manchester City defender is being monitored by Barcelona and in spite of the worrying head injury he sustained last season, the teenager has been impressive at the Etihad.

15 - Julian Draxler

Draxler admits injuries have thrown him off his game and he even considered joining Leeds in the summer. 

14 - Javi Martinez

Bayern are willing to let the midfielder leave for nothing at the end of the season as he's only made one start in the Bundesliga this season. 

13 - Hakan Calhanoglu

Manchester United are among the clubs looking to make their move, though he's still in contract talks with AC Milan. 

12 - Gio Reyna

Reyna is only 17 and is already drawing comparisons with Christian Pulisic as the next poster boy of the USA national team. The teenager was named as part of The Guardian's 'Next Generation' last year. 

11 - Jerome Boateng 

Joachim Low thinks he's finished, we disagree. Boateng is now 32 but his experience and winning mentality would be a huge asset to most sides. 

p1emd3n3lk1mlvu5uu0010gd2p7d.jpg

10 - Memphis Depay

With four goals in his seven games so far this season, Memphis is continuing his mission to prove United made a mistake letting him go. His unimpressive stint in England might put a few clubs off, mind. 

9 - Arkadiusz Milik

It's unlikely that Milik, who has been a reliable goalscorer at Napoli, will still be on the shop floor in the summer. Tottenham and Everton could swoop for him in January. 

8 - Georginio Wijnaldum

Now that Liverpool have so many options in midfield, Wijnaldum may start to look elsewhere. The Dutchman was linked with a move to Barcelona in the summer that failed to materialise. 

p1emd3o330jdlpriev11m6c1h4kf.jpg

7 - Gianluigi Donnarumma

The goalkeeper is still only 21, but there has always been a question mark over how long he would stay in Milan. The Italian has the potential to become one of the world's best. 

6 - David Alaba

Liverpool and Manchester City are among the clubs linked to the versatile Bayern Munich star. The 28-year-old has won no fewer than TWENTY-THREE trophies in Bavaria. 

5 - Angel Di Maria

The least popular man in Manchester, but Di Maria has proved he is still lethal with his impressive displays for PSG. The Argentine arguably doesn't get the credit he deserves in Paris. 

p1emd3ouv7p2er38ugul0a1gash.jpg

4 - Luka Modric 

The 2018 Ballon d'Or winner isn't necessarily on his way out of Real Madrid just yet. The Croatian is planning to extend his current deal and is reportedly even willing to take a pay cut to make it happen. 

3 - Sergio Aguero

Aguero's injuries are a sticking point and he's currently recovering from a hamstring problem. Manchester City's all-time leading goalscorer won't be short of suitors. 

2 - Sergio Ramos 

Ramos recently scored his 100th Real Madrid goal, a phenomenal achievement for a centre-back. The Spanish international would be a superb addition for anyone looking to win that little bit nastier. 

1 - Lionel Messi 

Of course. Messi will cost Barcelona a fortune when he leaves due to a bonus in his contract, but one lucky club will be able to land the GOAT for nothing.

p1emd3qklq1l981op1pcfnnb12r4j.jpg

What a shopping list we have here.

It's not too late for many of these players to sign new deals, but for many - including Ozil - there's no chance they'll be staying put. It just remains to be seen who takes advantage.