rule 18.07 states that if a player features in the first, second or third qualifying rounds of either the Champions League or Europa League, he can play for another club in either competition from the group stages onwards, provided his former employers do not reach the group stages themselves.
Found this on t'interweb that may help:
http://sport.stv.tv/football/clubs/...ity-of-cup-tied-players-in-uefa-competitions/
It's to prevent the chance of a player being up against a former club he's already played for in the same season.
Chelsea went down to the Europa League and could potentially face Liverpool,
Schalke stayed in the Champions League and it's no longer possible for Schalke and Spurs to play eachother this season.
But Holtby's former employers HAVE reached the group stage ???
We could meet this season if both team win there comps.
Schalke are still in the CL, thus he can play for us in the EL. Chelsea are now in the EL, so Sturridge cannot play in the EL.
Next season surely.