Doubt has been cast over Brisbane Lions No.1 ruckman Matthew Leuenberger's chances of returning in 2009 as his AFL club investigates a staph infection that sabotaged his season.

Leuenberger has pencilled in a round-21 return after overcoming the post-knee operation infection that stripped 11kg from his frame and delayed his comeback by months.

The Lions launched an inquiry after assistant coach Craig Brittain recently suffered the same post-operation staph infection just months after Leuenberger.

While Leuenberger has returned to training, Lions assistant coach Justin Leppitsch was cautious about predicting a return date for the young ruckman.

"He is a late-season starter if he does come back," Leppitsch said at Brisbane on Thursday.

"It's one thing to have recovered from injury but you also have to be fit for football.

"And that's the bit that takes an extra month of conditioning, running."

Lions CEO Michael Bowers said the club had begun testing at their Gabba dressing rooms to eliminate any possible in-house causes for the infections.

Just days after Bowers revealed Brittain was "struggling", Leppitsch painted a much brighter picture of the assistant coach.

"He's great. It knocked him about a bit, but he is on the back end of it now," he said.

Bowers said the club was in daily contact with Brittain.

He said the club had established an investigation team and was receiving support from Queensland Health, the Gabba's management Stadiums Queensland and St Andrews Hospital, where Leuenberger and Brittain both underwent knee surgery.

Bowers said the investigation team would have a "fully vacant environment for probes, medical tests and swabs, intensive cleaning and chemical treatments" with both the Lions reserves and AFL teams flying out for away games by Friday.

Mitch Clark has stepped up in the absence of Leuenberger and fellow ruckman Jamie Charman (ankle), drawing praise from the recovering big man.

"Despite only playing sparingly in the ruck for the first month of the season, Mitch has since shown that he can match it with the best in the business," Leuenberger told the Lions website.

"His role is likely to continue for the rest of the season as well with Charmo out until next year with an ankle injury while I'm still probably a bit over a month away from returning.

"He has played in every match for the club so far this season and is quickly becoming one of the teams most important players.

"It's been a massive effort particularly considering he managed only 22 senior matches in his first three seasons in the AFL.

"I think Mitch is really enjoying the responsibility and challenge of being the team's No.1 ruckman."

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