The Serbia international left Chelsea for Benfica in 2011 as part of the Blues' move to sign David Luiz but sealed his return to Stamford Bridge on Wednesday in a £20.75m deal.
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"I am very pleased that Nemanja is returning to Chelsea," Mourinho told the club's official website.
"He has grown as a player in Portugal and has become a fantastic all-round midfielder. I am sure he will become a very important member of the squad and help us to achieve our ambitions."
The defensive midfielder failed to impress in his first spell at the club, spending one of his three seasons with the west Londoners on loan at feeder club Vitesse.
Matic returns to compete with the likes of John Obi Mikel, Frank Lampard, Ramires and Michael Essien for a first-team place and he feels confident that he can make more of an impact this time around.
Matic told the club's official website: "I am very happy for this opportunity to be back at this club.
"I feel very good because of that and just want to say I will give my best to help the team and make the Chelsea fans happy."