Zaliukas on Thursday 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 Tynecastle 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 will be a tough task for a side which has seen its average age plummet in recent weeks.
He said: "Marius will be a big loss. He's the captain of the team and an influential player.
"There's 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."
Mullen has grasped the chance to make his mark in the team as Hearts have opted to cut their wage bill and utilise their youth-team options.
Other kids like Callum Tapping, Scott Robinson and Jason Holt have done likewise but Mullen knows that a winners' medal from next month's Scottish Communities League Cup final date with St Mirren will help them graduate from first-team prospect to established professionals.
"We young lads haven't really done anything in the game yet, we haven't played that many games so far," he said ahead of Saturday's Scottish Premier League meeting with Kilmarnock at Tynecastle.
"But I suppose getting a cup final win under my belt so soon would be a great way to kick things off. However, the final is still a few weeks away and the team hasn't been picked, so I don't even know if I'll be playing or not.
"That explains why it's so important to perform well over the next few weeks. Everybody would love to play in this cup final, so you need to try and keep hold of your place until then."