The 23-year-old hit the ground running last season following his arrival on an 18-month loan from Juventus, scoring five goals in 18 games as Spurs qualified for the Champions League.
This season has proved more difficult for both club and player, prompting suggestions that Spurs would not activate the £30.5million clause to sign Kulusevski permanently.
It is now understood that Spurs will sign Kulusevski, and Mason was full of praise for the young Sweden international when speaking to reporters on Friday.
"The general feeling is that Deki is a Tottenham player so those conversations, like many other conversations about other players, will come at the end of the season," Mason said.
"I love Deki as a player. I think he's got so much room to grow and be a top, top player, even though he already is a top, top player.
"At the same time when you play for a club like this, I feel it's important that you feel you have to earn the shirt in every moment.
"Naturally when you play for a club like this there's competition. At the moment, last week, you could see he was on the bench but he came on and helped the team get the result. That's important."