Buttner has struggled for first team action since joining United in August from Vitesse Arnhem.
"I had no preseason at Vitesse and so must again try to get my fitness to the required levels," he said. "But I think I'm doing well. It goes to show the level of difference, it cannot compare with the Netherlands. Everything goes faster. The tempo is higher and everyone is physically stronger.
"I sit every day in the weight room, which I did in the Netherlands. It is also mandatory at Manchester United.
"Alex Ferguson does not talk much, his assistants do."