Johnstone, who had served as United's third-choice option behind David de Gea and Sergio Romero, joined Villa on loan until the end of the season on Thursday, with Jose Mourinho keen for the 23-year-old to play first-team football.
Tottenham host the Championship side at White Hart Lane in the third round of the FA Cup on Sunday. Speaking at a press conference, manager Steve Bruce explained he will talk to his new player before making a decision.
Bruce said: "He has only trained on Thursday so far so I have it in my mind 'is he ready for Sunday?'
"Is that fair on him? Or do we let him train with the boys and get used to his surroundings for the next 10 days ahead of Wolves a week on Saturday.
"I will see how he is and see what he thinks. Whether I throw him in against Spurs... I am yet to decide. But certainly I have brought him here to play. But he has to play on merit – we all understand that."
Johnstone was given the green light to move to Villa Park after United confirmed the return of Joel Pereira to the club. The Portuguese goalkeeper enjoyed a fine first-half of the season on loan at Belenenses, but has now returned to Old Trafford, effectively taking Johnstone's place as the club's third-choice goalkeeper.
"I'm really delighted to be here and I'm happy that it's all sorted," Johnstone said after his move was confirmed. "I can't wait to work with Steve Bruce too. I've heard very good things about the manager. You can see his track record. He's good for the club and I can't wait to get going now."