The Sun is reporting that Manchester United are looking to raid the defending Premier League champions in successive transfer windows.
The same article states that Mata is expected to leave Manchester United at the end of the current campaign due to a lack of first-team opportunities.
According to the same story, Chelsea FC would demand a transfer fee in the region of £30m to sell the Brazil international at the end of the season.
The Blues signed Willian in a £30m deal from Russian side Anzhi Makhachkala in the 2013 January transfer window.
Jose Mourinho helped to get the best out of the South American at Stamford Bridge after Willian played an important role in their title success in 2014-15.
Manchester United are in second place in the Premier League table and 12 points ahead of Chelsea FC, who are toiling in fifth spot.
Willian has won two Premier League titles and the League Cup during his four years at Stamford Bridge.
The Blues can still finish the season with some silverware if Chelsea FC beat Southampton to reach the FA Cup showpiece.
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