Zaliukas this week underwent surgery on the ankle ligaments he tore in a tussle with Dundee United's Johnny Russell last weekend and will miss the rest of the season.
McGlynn has already admitted that the out-of-contract centre-back has "possibly" now played his last game for a club he first joined in 2006.
The manager could turn to either Darren Barr or Danny Wilson as he looks for a new defensive partner for Andy Webster - but 19-year-old Mullen would not be surprised if one of his teenage team-mates was also called upon.
However, he expects finding a new source of maturity and influence amongst a squad which has seen its average age plummet in recent weeks will be a tough task.
He told Press Association Sport: "Marius will be a big loss. He's the captain of the team and an influential player.
"There are a lot of young players in the team now and he helps us all through the games with his experience, so not having that there is a blow.
"And being the skipper, he's also a big figure in the changing room and one of the most vocal lads in the team. Having to do without him will be hard.
"He obviously plays a huge part for us and we will miss him. But I'm sure someone can come in and do a similar job as well. It could be another chance for one of the young lads."