Ole Gunnar Solskjaer admitted "the next few years are going to be massive" for Manchester United as they seek to reestablish themselves among the European elite.
United were comprehensively outclassed at the Nou Camp as they crashed out of the Champions League at the quarter-final stage, with Barcelona's 3-0 win on the night sealing a 4-0 aggregate triumph.
Lionel Messi's first-half double - the first a trademark finish, the second courtesy of a blunder from David De Gea - plus Philippe Coutinho's 61st-minute wonder goal gave a stark reminder of the difference between the two sides.
Solskjaer will now turn his attention to the domestic front, where United will vie for a top-four spot in the Premier League, but the Norwegian knows he faces a major rebuilding job this summer and beyond.
He said: "We did well to get here and you could see the difference between the two teams.
"We know there's work to be done. I've said all along this isn't going to change overnight and the next few years are going to be massive for us to get to the level where Barcelona and other teams are at the moment.
"We have to look at ourselves, we know we have a job to do and we are getting on with it.
"We need to get the best out of each other, create an environment of top, top, top-class attitude, world-class attitude every single day. Because we've got good players and I've got good players to work with.
"We've done really, really well to get to the quarters and challenge for the top four.
"So we've got a rebuilding job but it starts with the coaches and players, and of course one or two additions to the squad will happen in the summer."