That is according to pundit and former Liverpool defender Steve Nicol, who believes a major improvement is needed to the squad.
Mourinho won three pieces of silverware in his first season in charge at Manchester United, qualifying for the group stage of next season's Champions League by winning the Europa League last month.
The Red Devils finished sixth in the Premier League though, 24 points behind winners Chelsea.
Chelsea won the league after finishing the previous season in 10th, but Nicol cannot see a similar path for United unless they make many signings.
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Steve Nicol believes Man Utd need up to five signings to win the Premier League next season
To go from sixth to first you're talking major and that major is half a team. Chelsea were the Premier League champions 18 months before
Steve Nicol on Manchester United
"Five or six (players needed), yeah," he said on ESPN FC.
"Absolutely that many. This is a team that had to qualify for the Champions League through the Europa League.
"What did they finish in the league? Sixth? To go from sixth to first you're talking major and that major is half a team.
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"Chelsea were the Premier League champions 18 months before.
Shaka Hislop and Steve Nicol both believe Man Utd need major improvement this summer
"So they actually had the pieces in place, they just needed somebody who could put them on the field that would work and he (Antonio Conte) did that. Jose didn't have that luxury."
Former West Ham goalkeeper Shaka Hislop agrees and thinks the addition of Champions League football will make it an even tougher task.
He said: "Manchester United also have to deal with the Champions League this season, so if you're talking about being Premier League champions, you need a depth in squad to continue to fight on both fronts, which clearly they didn't have (last season)."