The rules are a little complicated but i understand that if he played tonight and spurs went through, he wouldn't be able to represent anyone else until Feb when you register the squad again. The so called groin injury seems just a wee bit suspect to me.
I thought that too but this is what I found when I looked at the rules: "17.07 Excluding the three rounds of the UEFA Intertoto Cup and the UEFA Super Cup, and subject to paragraph 17.18 below, a player may not play UEFA club competition matches for more than one competing club in the course of the same season. A substitute player who is not fielded is entitled to play for another club competing in the same season’s UEFA club competitions, provided that he is registered with the UEFA administration in accordance with the present Regulations. One player from the above quota of three who has played UEFA club competition matches for another competing club in the current season may exceptionally be registered, provided that the player has not been fielded: − in the same competition for another club − for another club that is currently in the same competition" He can't play in the same UEFA competition for two clubs in one season but he can if they're different competitions.
Basically, it means that you can play for a team in the Champions League and then play for another team in the Europa League or vice versa. You cannot play for two teams in the same competition and you can't play for a team in the CL and then play for a EL club, if your original club gets dropped into the EL too. None of that would apply to Modric, unless Chelsea are planning on finishing third in their group.
