The Red Devils are close to completing the signing of the English international from Borussia Dortmund, per various reputable media sources in England and Germany.
Sancho will reportedly sign a five year contract, with the option of a sixth year.
"They'll still be inconsistent," Barnes told bettingexpert.com.
"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. They'll be in the top four but they're not consistent enough and they haven't got the right balance between attack and defence to compete with Manchester City or Liverpool.
"You have to take signings when you can get them. As much as you can say, 'let's not buy Sancho and get a centre-back', if you don't sign a centre-back you've missed the chance to get a really good player.