United earned a draw against Benfica in their last European match, but their next two fixtures are against the Swiss side and Romanian outfit Otelul Gulati, and Fletcher knows that the Champions League can often deliver a shock.
"You have got to be cautious about unknown quantities," the Scot told Sky Sports News.
"Obviously we went to Benfica and got a draw, but the two we face in our next two games are a little bit unknown. Some people expect you to just walk over teams like that and win comfortably.
"It's never as easy as that though. We are playing in the Champions League and we know from past experience how difficult it is."
The Red Devils are currently in second place in Group C, while Basel sit top of the table after their 2-1 win in Romania.