The former Everton manager points to the likes of Bayern Munich, Barcelona and Real Madrid as having more exceptional squads as he prepares to take on Shakhtar Donetsk
The Red Devils got off to a good start in their group-stage campaign, beating Bayer Leverkusen 4-2, but the Scot is unsure about how they will compete when they face bigger teams.
"To win the Champions League you have to have five or six world-class players," he remarked to the press after his side's surprise 2-1 defeat to West Brom on Saturday.
"If you look at Bayern Munich, they have five or six really world-class players and, to win it, you might have to have that in your squad.
"Look at Barcelona, who had it in the past, and Real Madrid have maybe got it now.
"That's the level you have to get at to win it. We've not got that yet. What we have got is experience and several players who are in that category or close to it."
United travel to Ukraine on Wednesday to face Shakhtar Donetsk in the second game of their Champions League group.