The Belgian was one of seven players the Manchester United boss drafted into his starting XI following the Europa League win against Rostov on Thursday.
Fellaini impressed as a substitute for the injured Paul Pogba in that game and continued his good form by opening the scoring in the 3-1 victory at the Riverside.
And Mourinho believes his fringe men have quietened their critics.
"The meaning is great for the group," Mourinho told MUTV. "So many players not playing.
"Obviously many people thinking that today would be very hard for us but the answer was magnificent.
"I was expecting this kind of answer.
"I knew that Fellaini played the other day against Rostov for 40 minutes and he was amazing so his form is high.
"Ashley Young played in Rostov and Stamford Bridge.
"I knew that the guys were ready to answer."
