Costa has scored four times this season and just seven goals since January, when his prolific start to life at Chelsea after his move from Atletico came to a halt.
The 27-year-old was hindered by suspensions and a niggling hamstring injury, while he admitted reporting for pre-season training last summer carrying additional weight.
The Brazil-born Spain striker has taken time to show any menace and often appears too pre-occupied antagonising defenders to focus his energies on scoring goals.
He has apologised for his petulant bib throw in the direction of Mourinho when an unused substitute at Tottenham Hotspur and was restored to the starting line-up at Porto, after which he spoke of a possibility of returning to the Vicente Calderon Stadium as rumours continue he has not settled in London.
"He didn't say that. He said in football you never know," Mourinho said.
"Felipe Luis was one example. He could also say Fernando Torres, another example.
"Atletico is a great club. Madrid is a great city. La Liga is a great league.
"So why can't a player at 27 years old think that in football it's possible to be back? I don't see that as a problem."
Mourinho was asked if Costa could be a Chelsea great, like Didier Drogba.
"Drogba was here for 10 years. I don't see Diego playing till he is 38," Mourinho added.
"He is happy here. He likes to be here. He's another one who has, I think, three more years (on his) contract.
"I see him staying at Chelsea for three more years, no problem. I see that."
Mourinho first arrived at Chelsea in June 2004 and settled quickly, having, he says, earned his move from Porto.
But he believes now it is 'too easy' for foreign managers to be appointed Premier League bosses, stifling the development of native talent.
"When you have English clubs (making you an offer) you don't think twice," Mourinho said.
"You come to the country number one in European football, to the championship number one, and you feel that you have to deserve to be here. I think I did enough to deserve to be here.
"At this moment it's too easy. The number of foreign coaches in the Premier League, even in the Championship and maybe in League One - I don't know - is too big compared with the number of English, or in this case British, managers.
"Yes, I speak against myself, but I think it's true.
"I am not British, but I feel sympathy (for British managers)."
Mourinho pointed to his native Portugal, where 15 of the 18 Primeira Liga managers are locals.
He believes that is his legacy, after his hugely-successful stint with Porto, which culminated in the 2004 UEFA Champions League win and a move to Chelsea.
"Some people say that - and it's true because something changed in the last decade - that it's a bit of an effect of what happened with me," Mourinho said.
"People start believing and start giving chances and they were doing well. Then comes another one (manager), then another one, then another one."
Mourinho says there are 'lots of good coaches' in England and it is down to them to take their opportunity.
"When you have the chance, when you have the real one, the big one, the one where can make a big impact by winning something, by having the chance in front of you and grabbing it with both hands - it's what they have to do," he said.