The Red Devils completed a surprise £40m deal to sign the Serbia international from the 2017 Premier League champions back in August.
Matic’s exit left a void in the middle of the park at the west London side and France international Tiemoue Bakayoko struggled to adjust to England.
The Serbian star was a key player at Manchester United under Jose Mourinho, despite the Red Devils finished the season without silverware.
Matic and Manchester United lost 1-0 to Chelsea FC in the FA Cup final at Wembley after Eden Hazard’s winner from the penalty spot.
Former Arsenal star Merson slammed Chelsea FC’s decision to sell Matic to their bitter rivals in the 2017 summer transfer window.
“Matic – how Chelsea let him go is beyond a joke,” Merson told Sky Sports.
Chelsea FC finished in fifth place in the Premier League table after a 3-0 loss to Newcastle United at St James’ Park in their final game of the season.
The Blues lost 4-1 to FC Barcelona in the Champions League round of 16 to make a premature exit from the competition.
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