That is according to BBC Radio 5 Live pundit and Stoke man Charlie Adam.
Manchester United finished a massive 19 points behind Premier League champions Manchester City last term.
Jose Mourinho has secured deals for Fred, Diogo Dalot and Lee Grant this summer however, he’s still chasing moves for at least two more players.
Harry Maguire and Ivan Perisic are high on his target list but with the transfer window closing next Thursday, time is not on Mourinho’s side.
Midfielder Adam believes United need to make more than two signings – fast.
“Five, easily,” said Adam, when asked how many players Mourinho realistically needs to challenge for the title.
“They need a new left-back, a centre-half, a holding a midfielder.
“[They] need to get the best out of [Alexis] Sanchez somehow. Do they need to get another behind [Romelu] Lukaku?”
Former West Ham star Matthew Upson agreed with Adam’s take on Mourinho needing someone to play off Lukaku.
“I think so because when you talk about [Marcus] Rashford and Sanchez, it’s a very distinctive role, it’s a key role,” said Upson. “You are leading that line, you are the focal point.
“When you’re constantly playing, darting around the spaces and that is your position, to go and switch to that, it’s not easy.
“I saw Rashford try and do it at Brighton away and it was a disaster for him.
“It’s such a focal part in how they play in terms of holding the ball up but you just feel that he needs another player behind Lukaku to lead the line.”
Man Utd transfer news – the window so far
Players in: Fred (£47m, Shakhtar Donetsk), Diogo Dalot (£19m, Porto), Lee Grant (£1.5m, Stoke).
Players out: Dean Henderson (loan, Sheffield United), Joe Riley (undisclosed, Bradford), Sam Johnstone (£6.5m, West Brom), Daley Blind (£14.1m, Ajax), Matty Willock (loan, St Mirren), Cameron Borthwick-Jackson (loan, Scunthorpe), Max Johnstone, Jake Kenyon, Ilias Moutha-Sebtaoui, Devonte Redmond, Theo Richardson, Charlie Scott (all released), Michael Carrick (retired).