SYDNEY defender Aliir Aliir has been dropped for Sunday's critical clash with the Brisbane Lions for disciplinary reasons.

Aliir, who has spent the past two games in the NEAFL to work on his form and defensive work, failed to attend the Swans' final training session at the SCG on Saturday, and Swans general manager of football, Tom Harley, said the defender's actions were unacceptable.

"It is disappointing and unfortunate that Aliir comes out of the side," he told the club's website.

"We had a training session yesterday and it is disappointing that Aliir didn't attend. We can't stand for that as a football club, so he has taken full responsibility for that."

Harrison Marsh has come in to replace Aliir after recovering from a hamstring strain, and Harley said the West Australian was ready to resume his role in the back half.

"It was a very minor hamstring strain and we were very cautious with it given he has had some soft tissue history, but we are very confident based on the work he has done this season that he can play a role for us this afternoon."