Premier League sides are allowed to register 17 non-home-grown players in their squad submission for the competition, and City have just hit that number with the acquisitions of Sagna, Fernando and Cabllero. As none of those 17 are under 21, they can't move them into the unlimited places in the under 21 list. So City will need to sell or loan someone out if they want to buy another (over 21) non-home-grown player and play him.
Last season, Salah could hide in Chelsea's under 21 list but can no longer do that this season. This takes Chelsea's number to 15 after their disposals (Hilario, Luiz, Essien, Ba, Eto'o) and acquisitions of Luiz and Costa. Fabregas counts as home-grown due to the time he spent in the Arsenal academy.