The 30-year-old's is still deliberating whether to agree fresh terms at Old Trafford, having entered the final weeks of his existing deal.
Fellaini appeared to criticise United earlier this week by suggesting that he 'should not have to ask 10 times' for a new contract, while also warning that it would cost the Red Devils £50m to replace him.
In response to the Belgian midfielder's quotes, Mourinho has hit back by claiming that Fellaini could have put pen to paper at the start of the campaign.
"He let his contract run until the end. You cannot say that it was the club," he told reporters. "If there were no offers, then we could say the club let it go until now. He had offers. And he had offers in a progressive way.
"Of course, the offer he has now on the table is a better offer than he had six months ago or eight months ago. It's not frustrating for me because that's football.
"It's a process and he is using the fact that he is at the end of his contract to let it go. Is it a decision good or bad... but it is his decision and we have to respect that."
Fellaini, who has scored five times in 22 appearances for United during the 2017-18 campaign, is reportedly wanted by Paris Saint-Germain and Juventus.