The Europa League squad changes were announced on Tuesday, and they were mildly confusing. We decided to spell things out real quick. Milos Veljkovic is in, but Nabil Bentaleb is not. Don't worry, it makes sense.
Bentaleb, 20, was signed by Tottenham Hotspur in January of 2012. The Europa League rules state that a player who is under 21 and has been with his club for at least two years does not have to be registered on the 25-man 'A' list. They're part of a 'B' list and can be named in the squad at any time. Bentaleb is now part of that 'B' list.
Bentaleb was not registered for the Europa League at the start of the competition, meaning he was not eligible to play in the group stage. That's why the likes of Kenny McEvoy, Shaq Coulthirst and Laste Dombaxe made appearances in the squad during the group stage, but not Bentaleb.
Veljkovic was the one announced addition to Spurs' Europa League squad on Tuesday, with Simon Dawkins (and Lewis Hotlby) being unregistered. Dawkins counted as a locally trained player for Spurs, one of eight that they need to register. Bentaleb, though he has been at the club long enough to qualify as an under-21 player, has not been in England long enough to qualify as a locally trained player.
It's a bit tricky if you don't know the rules exactly, but there's nothing to be concerned about. Bentaleb is eligible to play in the Europa League, along with all of the registered players. You can find a full (now slightly outdated) list here.