Selling and replacing the Englishman must be a priority this summer for United manager Ole Gunner Solskjaer, who outlined his transfer plans in his post-match press conference.
Barcelona's win against Manchester United has also seen them take another step closer to winning the treble this season, but Messi stressed that the club had to take the rest of the campaign in bitesize pieces.
Barcelona's Argentinian forward Lionel Messi celebrates after scoring during the UEFA Champions League quarter-final second leg football match between Barcelona and Manchester United at the Camp Nou stadium in Barcelona on April 16, 2019.
"We have to aspire to get to that level of Barcelona, we can get there but we have loads of work to do". 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.
Jesse Lingard's 79th-minute shot from distance was a rare second-half highlight for United, while the introduction of former Barca star Alexis Sanchez for Lingard a minute later brought a round of applause from the home fans as they recognized the arrival of a former player. It's a big week and we have to get ready for that week because (they are) three big games. I wouldn't lie if I said we do a super special preparation. "I dont mind if we score the victor in the 93rd minute".
"The quality of their finishing was absolutely outstanding".
"If we had got the first goal then it might have been different but when they score two goals in four minutes it's just game over".
"The attitude was right, we were fighting against a good team and good players out there".
He added Messi's majestic display underlined why is he regarded as the best player in the world alongside Cristiano Ronaldo. "That is what you have to do to be able to continue in the Champions League and that you consider us more or less favourites", he said.
"You can prepare as much as you like, if you give him space and time around the goal he'll create and score goals, he's a fantastic player".