Theo strained his quad during Roar's 2-1 win over Central Coast Mariners at the weekend and has struggled to train since, taking no part in Thursday's session at Griffith University.

The 34-year-old was sent for scans earlier in the week after initially complaining about a hip flexor issue, with Aloisi hopeful the veteran shotstopper will only miss one game.

"I can't put a date on it because I'm not a doctor. I wish I was," Aloisi told reporters.

"We're pretty sure he should be back for the next game (against Adelaide United).

"It's not a major one, but it's one that if he does play, it could turn into a major one."

It's a worrying development for Theo, a five-time A-League championship-winning goalkeeper, who was restricted to just four appearances last season as he suffered a broken wrist and then quad and groin injuries.

Theo had appeared to shake off his injury woes and started in both of Brisbane's wins over Western Sydney Wanderers and the Mariners, but his misfortune has opened the door for fellow goalkeeper Jamie Young to stake his claim for the No.1 spot.

"I've said it all pre-season - we're lucky to have two great goalkeepers," Aloisi said.

"Jamie will just slot in and I'm sure he'll do a great job.

"He's been training really well, even when he didn't start the first two games he didn't put his head down.

"That shows the rest of the players - you keep working because you don't know when your opportunity's going to come up."

Aloisi flagged the possibility of further changes to his side, with fullback Jerome Polenz and attacker Dimi Petratos looming as possible inclusions after training with Roar's first XI in drills.

"With all my players, positions are up for grabs," Aloisi said.

"It's not only that I watch every game closely but I watch every training session closely and analyse the opposition a lot.

"If I think certain players will do better against certain opponents, then I'll choose those players.

"I'm not a believer in staying with a winning side because you don't change a winning side."