Left-sided John Jackson has already admitted that he sees it as 'make or break' for him at the club while the likes of Lee Barnard, Rohan Ricketts, David Galbraith all fit into the same category.
"It's a big, big year," said Colin. "We've large numbers this season and I can almost guarantee we won't have this number next season.
"That's not a financial thing - we'll get to the end of the season and a lot of them will have to be making a mark in the first team squad or out on loan in league football.
"They don't get too much more than this year to make inroads into the first team squad and if they haven't, they probably won't be here for another season."
And it will be tough to make that impression.
Colin explained: "Even without the lads from the first team it all means healthy competition for places and the the chance to adapt formations.
"They will have to do well and if they do that in pre-season, it will be an indication of who will be in the frame to start at Leicester.
"When the seniors come down it will be even tougher for them to get a game but they have to make it as hard as possible to leave them out."
Colin Calderwood has stressed that it's a vital season for some of the youngsters at reserve team level.