Benfica have suffered back-to-back defeats to CSKA Moscow and Basel in Group A, while United sit top of the table after two comfortable wins.
However, ahead of United's trip to Portugal on October 18, Mourinho has insisted that their upcoming opponents will provide a greater test than the other sides in the group.
"The defeat suffered in Switzerland (Basel won 5-0) didn't change my opinion that Benfica are our main opponent in this group stage of the Champions League," the 54-year-old told Record.
"Benfica's bad start in the Champions League doesn't change the approach to the double-match that is at the door. They are better than CSKA and Basel."
United will effectively secure their place in the last 16 if they can record two victories over Rui Vitoria's team.