The Argentina midfielder picked up an injury at the end of the last campaign that meant he was delayed in beginning full pre-season work with the squad this summer.
He returned to match action with 25 minutes against Yokohama F. Marinos in Japan last week and then played just shy of an hour in the first of two friendly wins over Athletic Club on Monday.
“He will be in the squad [on Sunday],” Slot said of Mac Allister at his press conference to preview the Wembley curtain-raiser versus Crystal Palace.
“He was out for two months I think, or longer because we didn’t play him in the last four games of the season. He trained with us since a week now, played 30 minutes, 45 minutes.
“So that’s not a situation where you would start a player [and] play him for 90 minutes. Starting is possible but definitely not for 90 minutes.”
On Joe Gomez’s fitness, meanwhile, Slot said: “Unfortunately he has a minor injury at the moment but he will be back with the team very soon.”