Reports in Germany have claimed that United are preparing to table a big-money move for the forward.
"It's not my decision," Lahm told reporters in Shanghai, where Bayern are preparing to face Inter Milan in a pre-season friendly
"Things can happen very quickly in football
"I certainly can't provide any guarantee for Thomas Muller (staying)."
Lahm did insist there was no chance of him leaving Bayern, though.
"That's not going to happen with me," he said
"I'm pleased that I've been able to spend practically all my career at home
"It's great to be in the same club for so long."
And Lahm is not concerned about Bayern, who have dominated German football for the past three years, losing their domestic dominance
"We're still a top club," he said
"We've reached at least the semi-finals of the Champions League in recent years and we have a top team who would be playing at the top in any league.
"We certainly don't need to have any fear."
Source : PA