After being left out the squad altogether for Spurs' last two matches against Lokomotiv Plovdiv in the Europa League and Southampton in the Premier League, rumours have intensified that the Englishman could be set to leave the club.
Paris Saint-Germain were previously willing to provide him with a route out of London on loan, but with no developments on that front it appears that Dele will remain with Tottenham.
Mourinho, who described Dele as a "f*cking lazy guy" in training during Amazon's All or Nothing Spurs documentary, is convinced he will stay.
"I believe [he will]. I can even say that I'm convinced that he will be," the Portuguese told Sky Sports when asked if Dele will remain where he is.
The former Chelsea, Man Utd, Inter, Real Madrid, and Porto boss encouraged Dele to become more consistent if he is to realise his potential.
"We all know since Dele arrived from MK Dons he had ups and downs, he had fabulous periods and he also had other periods where he disappeared from top-level performances. And with me it's no different than before," Mourinho added.
"He's had moments where he was so good, but he also had moments where he was not so good.
"I think this lack of consistency, it's normal that when you are in great form you play because you deserve it and your production is big for the team, but when you are not in a good moment you don't play - and it's as simple as that.
"The players have to prove on the pitch with their performances; they have to prove how good they are in that specific moment."
Dele has been included in Tottenham's squad for their Europa League qualifier against Shkendija in Macedonia on Thursday.