The club are understood to be paying him around £230,000 a week to play at Old Trafford, meaning the one-year deal will cost United £12m, even if there was no transfer fee.
His four-year deal at PSG expired in June and the Swedish striker was able to fulfil a lifelong dream by signing for a Premier League club.
Henrikh Mkhitaryan, Borussia Dortmund, £26m
With only one year left on his deal, Dortmund found themselves forced to cash in on their star forward.
The 23-year-old left Old Trafford four years ago on a free transfer after his agent Mino Raiola brokered him a deal at Juventus, where he was given the chance to shine in the first team.
And now, with numerous club and individual trophies to his name, he has returned to Manchester to join a stellar cast of Mourinho, Ibrahimovic, Rooney, Martial and Mkhitaryan.
OUT (£8.5m)
George Dorrington, Huddersfield, free transfer
Jose Mourinho puts Manchester United stars through their paces in pre-season training
Nick Powell, Wigan, free transfer
Powell made nine appearances in all competitions for United and was sent out on loan three separate times.
And he failed to improve sufficiently to win a spot in the United first team but has now be given a permanent contract by Wigan, where he scored 12 goals in the 2013-14 season.
Oliver Rathbone, Rochdale, free transfer
Joe Rothwell, Oxford United, undisclosed
GETTY
Will Keane now plays for Hull having left Manchester United for £3m
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Guillermo Varela, Eintracht Frankfurt, loan
Victor Valdes, Middlesbrough, free transfer
The former Barcelona found himself frozen out of the United squad under Van Gaal after allegedly refusing to play in a match for the reserves.
His 18-month contract expired over the summer but he was quickly snapped up on a two-year deal by promoted Middlesbrough, managed by fellow Spaniard Aitor Karanka.
Donald Love & Paddy McNair, Sunderland, £5.5m
Former United boss David Moyes used his contacts at Old Trafford to secure a deal to strengthen his Black Cats squad considerably.
However, neither defender played under Moyes at United - Love in fact has only made two senior appearances for the club, his debut coming against Sunderland at the Stadium of Light.
Cameron Borthwick-Jackson, Wolves, loan
James Wilson, Derby, loan
GETTY
Cameron Borthwick-Jackson is out on loan at Wolves having been deemed unneeded at Manchester United
Tyler Blackett, Reading, loan
Andreas Pereira, Granada, loan
Will Keane, Hull, £3m
After loan spells at Wigan, QPR, Sheffield Wednesday and Preston North End, Keane finally left United for good to join promoted side Hull.