Benzema scored in the 1-1 draw last month and has been linked with a summer move to Old Trafford for a fantastical figure of about £45 million.
Ferguson said: "He's the goalscorer in the team, him and Juninho from set-pieces. They are a 100 per cent threat to us and we have to be aware of that. Benzema plays as a lone striker and he's a goal threat. The manner Benzema took his chance tells you he can score out of nothing, and you worry about players who score out of nothing."