Barnes reckons he knows why United have failed to challenge for top honours.
Speaking to beIN SPORTS, Barnes said: "If you look at Manchester United now, and it's still a huge club, still got great players, but I don't think they feel the responsibility of having to win.
"Speak to Man Utd fans now and they're going 'you know we're going to finish fourth, we can finish fourth' and they will be happy with that because they're in the Champions League and I think that's the big difference."
Justifying his opinion on United, Barnes explained: “I see a huge similarity between Manchester Utd and Liverpool, huge similarity and I'll tell you in what way.
"In the way when I first went to Liverpool, they were a successful team before I was there and the demands to win and the pressure to win was the most important thing, regardless of whether they had superstars Ian Rush, Kenny Dalglish and they had won everything.
"The demand to win everything was there. Then when all of a sudden, Manchester United, early 90s, and things had changed.
"If you look at the way the players were after say 1992/93, Liverpool still was a huge club, and it seemed that the players there were happy to be at Liverpool without necessarily recognising the responsibility to win.
"So when we were finishing 3rd in the league, 4th in the league, we were still Liverpool players, bigger than 99% of the teams, even if they finished above us, Arsenal whoever, but we were the Liverpool players without the demands to win."