Alli had hoped to join Paris Saint-Germain on loan in the January transfer window -- reuniting with former Spurs boss Mauricio Pochettino -- but chairman Daniel Levy blocked the move amid concerns over squad depth and a desire to see the player succeed in north London.
The 24-year-old has started just six games across all competitions this season but Mourinho confirmed he sat down with Alli yesterday to discuss his future and believes there is a key role for him to play once recovering from what the Spurs head coach has described as a "tendon" injury.
Asked what Alli had to do now he is staying at Tottenham, Mourinho said: "He needs to recover from his injury, he needs to be back to training and he needs to have a fresh start because in the last few weeks, he couldn't even train.
"So that's the most important thing. I had a good conversation with him yesterday. We spoke around this question that you ask me and I think we found very common ground, let's say. It is an important period of the season for the team, it is an important period of the season for him also.
"We need him. We need a good Dele Alli. We are just waiting for him to be back to normality, to be back to training with the team. It is not just Dele, [Harry] Kane, Gio[vani] Lo Celso, we are speaking about players that without all three, we miss a little bit of creation, some magic around the attacking areas so we need him back. I made that very sure to him.
"A conversation doesn't make miracles. A conversation doesn't put a player in great form or without any limitations in terms of fitness but I believe the conversation was maybe the extra motivation he needed to be back as soon as possible, to become available for the team.
"We play Europa League very soon, we are in the Premier League playing every week a couple of matches. We have the FA Cup, I don't believe he will on time for that one. We have a final to play in April so we need a good Dele back. And I believe that when physically he is ready, he will mentally be in a good place to go for it."
"He is happy with the progress, of course we are happy too," said Mourinho of Kane, who injured both his ankles against Liverpool last week.
"I think it is not a very optimistic thing to say that next week he will be playing. It will just be a consequence of his good evolution.
"We were a bit scared when everything happened but now we are much more positive and next week we play Everton midweek, [Manchester City at the] weekend, I believe for one of these matches he should be back."