The Red Devils have been chasing Sancho for two years and remain hopeful of finally completing the transfer this summer.
Speaking exclusively to bettingexpert.com, Barnes said: "Manchester United's problem hasn't been their attacking play and scoring goals: they've got Cavani, Greenwood, Martial, Rashford, Fernandes and Pogba, they're all good attackers.
"Sancho will add to that, but their problem is that they've not been consistent enough and they've conceded goals and lost matches because they're not strong enough defensively.
"It'll be a good signing for them - a young English player which is what you want in the squad. But I don't think it will make them challengers."