Gray players for reserves

When I started the game, before the regular season started, there were a bunch of gray players on my reserve team. But now that the season has started, they've gone away. There are still gray players in the Under 18 team.

I don't have enough players to field a full reserve team of players, so when they play a game they are exhausted for lack of substitutes.

What's the best way to solve this problem?

I know I can hire the gray players on my youth club for a minimal cost. Maybe if I bring them up to my reserve team, more gray players will backfill my youth club?

That's what happens when you don't have enough players for either the reserves or youth side. You could use any first team squad members that aren't getting a game or promote your youths. I would only sign a grey player if i'm very desperate to have a player as they are a waste of wage budget.