The Galatasaray coach believes his former club Manchester City, along with the likes of Chelsea, have overtaken United by bringing in new players to bolster their respective squads.
And the league table underlines that fact, with David Moyes' side 14 points off top spot, having lost as many top-flight games as City and Chelsea combined.
"Chelsea changed and bought some new players, City bought four or five new players," Mancini told BBC World Service.
"After many years, it's the time (for United) to refresh the team."
United's title defence is proving to be one of the most meek in Premier League history but Mancini refused to lay the blame at the feet of their Scottish manager.
Moyes was widely praised for the job he did at Everton over the course of more than a decade, although he never won a trophy, and Mancini is confident that the three-time LMA Manager of the Year will get it right at Old Trafford.
"I think there is a good chance for him - he worked for 10 years at Everton and he did a really good job," the Italian added.
"He is a good manager and I don't think he needs my advice."
One of the biggest challenges facing Moyes is the weight of his predecessor's legacy, with Alex Ferguson credited as the man who turned United into one of the most successful sides in Europe, and certainly England's most decorated club of the Premier League era.
And Mancini acknowledged that Moyes had perhaps the toughest of all acts to follow.
"Alex was at the top but for every manager it is difficult (to come) after him," he added.
"It's totally different to manage a team that should win every game, that every year (should) win a title or the Champions League."
Moyes, whose side won the Community Shield in August, will continue his quest for a first piece of major silverware when United welcome Sunderland in the League Cup semi-final second leg on Wednesday.
The hosts must overturn a 2-1 first-leg deficit if they are to set up a Wembley showdown with rivals Manchester City.
