The boss has been hampered by players suffering long-term injuries that has not given him the chance to strategically pace more experienced players through the campaign.
"It concerns me that we haven't been able to change," said Glenn.
"Round about Christmas time we needed our squad really strong, that was the plan, to be able to rest a few people then. We didn't get the chance and, at this stage of the season, we're in the same predicament.
"We haven't got the people here to give the likes of Gus, Teddy, maybe Les some rest period. That squad has been decimated right from the beginning of the season virtually with five or six long-term injuries.
"They've not come back and some don't look like they're going to be coming back in the near future.
"They have all had to play lots of games. That's been forced upon us through not getting Doherty, Carr, Freund, Clemence, Bunjevcevic back. They've all had long-term injuries that have held us back.
"It's not like two-week injuries we've had. It's six or seven players out for month after month and they're not ready to come back.
"There is no date for Stephen Carr at the moment. Stephen Clemence will play for the reserves again at Charlton, but the other little bunch aren't ready."
A boost for Glenn will be that Gary Doherty came through a half against Charlton on Thursday, which will increase his options in the coming weeks.
Glenn has admitted his concern over the fact he has been unable to give players a rest during the course of what has been a hectic season.