The Portuguese boss facilitated a move to ensure Borthwick-Jackson is able to get more regular action this season before returning to Manchester.
He rose through the ranks at Man United after joining at the age of six, he was then handed his first-team debut by Louis van Gaal last season and went onto make 14 appearances.
Speaking about his move, the 19-year-old hopes to pick up plenty of experience before he returns to Mourinho’s side.
“I had interest from Premier League clubs and other Championship clubs but I wanted to come here and help the club get back to where they belong, which is the Premier League.”
Borthwick-Jackson was given his opportunity because of injuries to van Gaal’s squad, while he usually featured at left-back, his preferred position is central defender and hopes are he’ll be able to play in his natural position.
He performed well for United last season and he’ll undoubtedly have a bright future at Man United, but with Luke Shaw‘s return from injury and United’s new signing Eric Bailly in central defence, he would have limited opportunities at Old Trafford.
The youngster is expected to make his debut for the Championship side on Tuesday evening against Cambridge United in the League Cup.