With leagues across Europe coming to an end, the focus will naturally shift towards the summer transfer window. It is expected to be a wary affair, with clubs careful about the financial instability caused due to the COVID-19 pandemic.However, clubs like Manchester United and Chelsea are expected to make big-money signings. Players like Jadon Sancho and Jan Oblak have been linked with moves away from their respective clubs.On that note, look at the ten biggest transfers that could happen this season.#10 Jan Oblak | Atletico Madrid to ChelseaAtletico Madrid's Jan Oblak has been linked with ChelseaIt is no secret that Chelsea are on the lookout for a goalkeeper. Having paid €80 million to sign Kepa Arrizabalaga from Athletic Bilbao only for him to endure a poor season, the Blues have decided to sort out the goalkeeping department.Recent reports have suggested that the club have highlighted Atletico Madrid shot-stopper, Jan Oblak, as an option. The Slovenian is regarded as one of the best goalkeepers in the world, having produced world-class performances consistently for his club.The 27-year old will not come cheap, however. Atletico Madrid are reportedly demanding €120 million for the former Benfica man and it remains to be seen if Chelsea cede to their demands.Chelsea are reportedly willing to offer 100m euros (£90.7m) for Atletico Madrid keeper Jan Oblak.Latest transfer rumours: https://t.co/1iW3hFksO9 pic.twitter.com/LSQ6NpVAE0— BBC Sport (@BBCSport) July 14, 2020Oblak has been linked with a move to Manchester United in the past. Arrizabalaga, on the other hand, is attracting interest from Valencia.Ajax's Andre Onana, Barcelona's Marc-Andre ter Stegen and Paris Saint-Germain's Alphonse Areola are other goalkeepers being considered by Chelsea.#9 Kalidou Koulibaly | Napoli to Manchester CityKalidou Koulibaly in action for NapoliSimilar to Chelsea, Manchester City need centre-backs. John Stones has failed to develop as expected, while Nicolas Otamendi is no longer reliable. That leaves Aymeric Laporte and young Eric Garcia as Pep Guardiola's only other options.Kalidou Koulibaly is one name which has been linked persistently. The Senegal international has earned the reputation as one of the finest defenders in Italy, with his physicality and ball-playing ability earning rave reviews.Napoli are said to be willing to deal for their star asset, but will demand a hefty fee for the 29-year old. A fee of €100 million has been touted, while Manchester City willing to pay up to €65 million.Kalidou Koulibaly's agent Fali Ramadani has had dinner with Aurelio De Laurentiis confirming the first concrete interest of #ManCity in the defender - and that City would not be willing to spend €75m, which the Napoli president wants.[via @DiMarzio] https://t.co/DQd304wyq4— Man City Report 💯 (@cityreport_) July 20, 2020Koulibaly has been linked with Manchester United, Chelsea and Paris Saint-Germain in the past. Bournemouth's Nathan Ake is another centre-back who has attracted interest from the Cityzens.Also Read: 10 footballers who are too good for their current clubs