"We are not getting carried away," he told PA Sport. "We cannot afford to think that just because we have had a good start it is going to be an easy ride. That is not the case.
"It could come back and bite us. We will go into this game [against Sporting Braga on Tuesday] like any other and just look to put ourselves in an even better position without looking too far ahead.
"It can be quite tough in the Champions League to judge [who will qualify from each group]. Some people might pick out two teams in a group and say they will be the top two and the others will struggle. Quite a lot of the time, that is not the case."
United will host Sporting Braga at Old Trafford on Tuesday.