Mac Allister struck deep in added time at the City Ground on Sunday to secure a 1-0 win for the Premier League champions.
The Argentina international sealed victory by pouncing on a loose ball from close range just minutes after he had a goal ruled out by VAR.
"I think what he needed is what he showed in the last six, seven or eight games - a run of games where he's getting back to his usual level that he showed so many times last season," said Slot.
"And he had that level also in the first half of the season, but it went a bit with ups and downs, as the team went in terms of performances with ups and downs.
"But I see much more consistency recently - not only in Macca's performance but in the team performance and many individual performances.
"But I think it's always nice for a player to score, especially if it's in extra time of extra time."
The Dutch boss, whose side are now just outside the Champions League places on goal difference alone following a run of poor results, added: "We needed this goal, we needed that win to be on the right side of things once in a while because we've been so, so unlucky this season.
"And for the first time, at least it felt to me for the first time, we've been a bit lucky this season."