He probably has players in his squad that are not signed yet. If you have a full 22 player squad, and buy an extra player, he will show up in your squad but you won't be able to use him. First you'll have to sell/sack another player, and then sign with the new one.

Same with youth players. If you have a full squad, when you get the youth you'll have 24 players in your squad, but you can't use the youth ones until you sign them, and you can't sign them before your sell/sack some other player.