McCarthy worked under Mourinho and was part of Porto's 2004 Champions League-winning side.
“This is the job that's going to change the whole Mourinho perception of not bringing enough young players through and only staying two or three seasons at a club because there's no bigger job than the United job," McCarthy told the Manchester Evening News.
"He can bring that Sir Alex Ferguson aura back to the club and for the long-term as well.
“He just has to prove to himself he can also go and build a small empire after what Sir Alex did.
"You can't replace a man like Jose so I really hope United fans embrace what they have now.
“I think this is the job he's always waited for."