The Red Devils have been linked with a huge bid for the versatile forward to test the German champions' resolve and the veteran defender says that anything could happen.
Reports in the German media have suggested that the Premier League side are preparing a second raid on the Bundesliga champions after securing the signing of Bastian Schweinsteiger earlier in July.
Full-back Lahm offered no assurances to Bayern's supporters that his fellow Germany World Cup winner would not follow Schweinsteiger to Old Trafford, where he would be reunited with former Bayern boss Louis van Gaal.
"In football you can imagine everything," he told reporters. "It's not my decision. In football, it goes so, so quickly.
"It's not always only up to the player. There are clubs who get players, players go. In football everything goes very, very quickly. I think you can never give a guarantee that it will work like this or like that.
"As a result, I certainly can't give any guarantee for Thomas Muller or talk about whether I can imagine [him leaving]. You can always imagine anything happening."