Martin O'Neill admitted Manchester United are a class apart from the teams Celtic beat on the way to the Uefa Cup final after his side were thrashed by the Premiership champions on Wednesday morning.
A 4-0 defeat at the hands of Sir Alex Ferguson's team brought to an end an impressive unbeaten run against Premiership opposition.
Despite the margin of defeat, O'Neill was not too despondent afterwards.
But he acknowledged that Celtic have some work to do before their first competitive game of the season, the Champions League qualifier against either Kaunas and Torshavn.
"Manchester are vying with Real Madrid to be the best team in Europe," he said.
"Their finishing was absolutely clinical but we didn't help ourselves with some very sloppy defending.
"It is only pre-season but we have been working for two weeks and clearly we still have plenty to do.
"If we had scored the penalty it would have gone to 1-1 and given us drive and enthusiasm to keep going but within a couple of minutes they had made it 2-0
and it was a long old battle after that.
"United have been competing in the Champions League for quite a few years and they have learned quite a bit from that sort of experience.
"We are improving and trying to get there but Manchester United are the benchmark."