United had a woeful campaign in defense last season, conceding 57 goals in the league alongside finishing with a goal difference record of zero.
They also recorded the worst points tally in the club's Premier League history (58 points).
The Red Devils have been linked with a number of defenders, including Ajax's Lisandro Martinez and Jurrien Timber (per CaughtOffside).
However, Saunders believes Erik ten Hag's side should have looked at signing 29-year-old English centre-back James Tarkowski.
The former Welsh star told talkSPORT (via HITC):
“I think Tarkowski should have been a good signing for Manchester United. Whether they don’t think he is good enough, but he defends for his life, blocks shots and puts his body on the line."
He was available on a free transfer with his contract having expired with relegated Burnley for whom he made 37 appearances last season.
Although Saunders doesn't regard Tarkowski as a top star, he believes the player would have benefited Erik ten Hag's defense, adding:
“He’s not a brilliant footballer, but stop the ball going into your net, for a start.”
United were heavily criticized throughout last season for their woeful defending with the likes of Harry Maguire and Victor Lindelof scrutinized regularly. Raphael Varane, who signed for the club last summer, spent a large chunk of the season out injured while Eric Bailly warmed the bench for the Red Devils.
Manchester United need to get the ball rolling in the transfer window
Manchester United are still yet to sign a player this summer as they head off for their pre-season tour starting on July 12.
The Red Devils might be on the brink of announcing Feyenoord left-back Tyrell Malacia as their first signing but there is still no movement in their pursuit of other targets.
Their top target is Barcelona midfielder Frenkie de Jong but there have been many twists and turns in their operation to sign the Dutchman.
Barcelona president Joan Laporta has stated that he is eager to keep hold of the 25-year-old but that may just be a negotiating tactic to drive up the player's price or to get the Dutchman to lower his wages at Nou Camp.
The Red Devils could have made an early signing by bringing in Tarkowski on a free transfer to ignite their transfer window.
Instead, their situation continues to get more complicated with star man Cristiano Ronaldo reportedly now keen to exit the club. He has asked Manchester United to let him leave should the Red Devils receive an acceptable offer.
The rebuild will understandably take time but the start has been rather poor.