The Reds take on the Spanish giants in their final tour game on Saturday at FedEx Field in Washington DC.
It's just two months since the last meeting between the pair when United lost out in the Champions League final at Wembley.
Sir Alex says Barca's achievements in the last few years - including another Champions League final victory over United in 2009 - have elevated them to another level which the Reds must also reach.
"In everyone's mind they're the best team currently in the world," the boss told reporters at his pre-match press conference in DC.
"I'm quite happy to be second place at this moment in time. Our challenge is to get to that level. It's not just about the football team though, it's the philosophy, what they believe in, how they coach, how they produce young players.
"We have the same drive ourselves to produce young players. If I thought they'd play all their young players on Saturday then I'd play all mine and it would be a fascinating game."
Evra too has great respect for Pep Guardiola's men, but also painful memories he would like to erase.
"I give Barcelona a lot of respect but [the defeat at Wembley] is fresh in my mind and I'd like to play against them again in the Champions League final and win. I've already lost twice and it's very painful," he insisted.