United will crash out of the competition if they lose in Switzerland against the team who held United to a 3-3 draw at Old Trafford in September.
And Ferguson's players will be wary of midfielder Frei, who has scored 15 goals this season, including two at Old Trafford .
Ferguson said: "Basel can score goals. The boy Frei is a finisher, there's no question about that. I just wonder how they'll approach the game.
"I can't see them going at us, to be honest. I think they'll try to wait and get something off the big striker and Frei. His goalscoring record is unbelievable at 32 years old."