United -- 21-time league and three-time European champions -- have struggled to recapture past glories since the retirement of Alex Ferguson in 2013, with David Moyes and Louis van Gaal failing to win major trophies at Old Trafford.
Van Gaal's men are struggling this season, fifth in the Premier League and six points adrift of the fourth and final Champions League qualification spit.
But with Mourinho unemployed following his departure from Chelsea in December, Deco - who won a Champions League title under the Portuguese manager at Porto - said United should do whatever it takes to install him as manager.
"Manchester United have to get back to the highest level and Mourinho is the man to do it," said Deco.
"Mourinho will bring confidence back to both players and fans.
"You cannot win anything if you have no confidence."
Mourinho has been constantly linked with the job since leaving Chelsea, with United manager Van Gaal under pressure to turn the club's torrid form around.
A 3-0 away win over League One outfit Shrewsbury Town in the FA Cup fifth round has released some of the pressure on Van Gaal.
But another loss to Danish champions Midtjylland - who beat United 2-1 in Denmark - and an early Europa League exit could prove terminal for the Dutchman.
United face a crucial Premier League match against third-placed Arsenal on Sunday.
