"It's not easy, Marco is coming back from injury, as am I, and it just doesn't happen like that," the Parkhead midfielder said.
"It's hard and I know how Marco feels because I'm not 100% myself.
"But when you are at a club like Celtic you just have to try and ignore that and make sure you are at your best.
"You need to make sure that you get goals and Marco will keep trying and do a good job for us."
Mowbray again defended Fortune, who struck five goals in 18 games under the Englishman on loan at West Brom last season.
"Fortune has had 10 weeks out," the Celtic boss said. "If you know anything about football, injuries knock the confidence of footballers and he needs to get himself back up to speed.
"It's going to take time, you can't just do that with just training.
"It's going to take games and that's why he is getting some minutes on the pitch at the moment.
"But I have every confidence in him. Once he is up and running he will be fine."