Tel was brought on for his debut in the first half of the 4-0 thrashing at Liverpool on Thursday in the semi-finals of the Carabao Cup after Richarlison sustained a fresh calf injury.
Richarlison has joined fellow forward Dominic Solanke on the treatment table and this will force Postecoglou's hand in attack ahead of a crucial FA Cup fourth-round tie at Villa Park on Sunday (Monday AEDT).
Tel only made four starts for Bayern Munich during the first half of the season, which resulted in the France Under-21 international seeking a January move and Postecoglou will try to manage the 19-year-old's minutes.
"For himself and (fellow recent recruit) Kevin (Danso) in many respects, it wasn't an indication of how we want to go about things, but just being out there, playing your first game, you get that out of your system," Postecoglou reflected.
"We're going to have to lean on Mathys a fair bit now, but he came here to play.
"His physical condition is not too bad. He hasn't played a lot but watching him in training and (Thursday) night, he was still moving quite well.
"We have to be careful with him also, but he came here to play and he'll get an opportunity to do that."
Postecoglou has limited resources for the quick turnaround to Villa with 12 players set to be absent, including Guglielmo Vicario, Micky van de Ven, Cristian Romero, James Maddison, Brennan Johnson and Solanke.
It has resulted in extended opportunities for talented teenage summer recruits Archie Gray and Lucas Bergvall, while 17-year-old Mikey Moore has also earned regular minutes this term.
The number of young players already being used by Postecoglou means he is loathe to put more in from the club's academy set-up, especially in such high-pressure situations.
He added: "We've already got young players out there. There's 18-year-olds, 19-year-olds, 17-year-olds out there.
"We saw what happened (Thursday) and our players are now getting criticised. Most of them will be able to handle it, but I wouldn't want to expose a young player to something like that, especially the way the world is today.
"You know we don't want to make determinations in a snap instance and destroy careers. I'm not going to expose young players who I don't think are ready for that, so we don't have the opportunity to rotate.
"I think it will be a similar kind of team that rolls out on Sunday. We've been doing this for quite a while now for the last three months and we have handled that pretty well at times and that's what we will try and do on Sunday."
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