Fellaini has endured a mixed time at Old Trafford since signing from Everton in 2013, but the 29-year-old has emerged as a trusted member of the senior setup in the North-West.
The player was absent from the 2-2 draw with Stoke City on Saturday and remains a doubt for the Champions League opener with Basel due to a calf injury, but Mourinho has suggested that the versatile Belgian will be involved if passed fit by the medical team.
The 54-year-old told reporters: "He's an important player for me - a lot more important than you can imagine.
"I feel weaker without Fellaini in my squad, doesn't matter if it's on the pitch or on the bench. If his conditions are improved, he will be selected, I need him."
Fellaini has netted once from four substitute outings in all competitions this season.