The Red Devils named the 53-year-old as their new manager on Friday morning after months of speculation linking Mourinho with the Old Trafford role.
Mourinho replaces Louis van Gaal after the Dutchman was sacked last week just days after guiding United to the FA Cup thanks to a 2-1 extra-time victory over Crystal Palace.
Ex-Chelsea chief executive Kenyon was responsible for bringing Mourinho to the Premier League in 2004 after the Portuguese manager swapped Porto for the Blues.
And the former United CEO suggested that Mourinho could prove the best signing in United’s decorated history.
“When I appointed him I was looking for someone who could bring a winning mentality to what was already a good team,” Kenyon told Sky Sports.
“What Jose brought was a winning mentality and that led to Chelsea winning their first title in 50 years that season.
“That’s the ability he has. He makes good players better. Everybody talks about new signings but the current players at Manchester will be better under Jose Mourinho.
“They’ll get a coach who brings new techniques, new developments and forward thinking. I think he’ll have an immediate impact. It’s United’s biggest ever signing.”
United finished in fifth place in the Premier League table behind Manchester City on goal difference after a disappointing top-flight campaign.
Mourinho’s side are facing the prospect of a second season in three years without Champions League football.
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