Saha, who helped United to a couple of Premier League titles during his stint at the club, thinks the French striker should be their main summer target.
The Red Devils have struggled to score goals this season, scoring more than one goal in only one Premier League game this season.
Speaking about United's attacking problems, Saha told Compare.Bet: "If you ask me, I would break the bank for Mbappe, but now it's too late.
"But, when you look at strikers, it's a dying breed. Lewandowski is a rare example of someone who is left, and I don't see many players that can easily fit up front.
"There is a need for somebody up front who is strong enough to hold the ball and to give time and space to the players like Paul Pogba and McTominay to create something in midfield to put passes together.
"You're asking a lot from young players because passing the ball around and having nice touches, it just looks too nice and it doesn't work."