Chelsea have completed moves for Arsenal forward Olivier Giroud and AS Roma defender Emerson Palmieri. But will both be able to play in the knockout stages of the Champions League?
To put it simply: YES!
UEFA Champions League regulations allow clubs to register three new players for the knockout rounds.
One of those players can have "been fielded in a UEFA club competition group stage match for another club in the current season" provided that player has not already played "in the same competition for another club," or for "another club that is currently in the same competition."
Giroud passes in each case -- he has only played in the Europa League, thus has not yet featured in the Champions League. And as Arsenal are in the Europa League, the clubs are not in the same competition.
Chelsea, of course, also signed Ross Barkley earlier this month. Although Everton were in the Europa League and he was named in their squad, a serious hamstring injury meant he did not play at all in the competition. Therefore he can be registered. If Barkley had played in the group stage, Chelsea would only have been permitted to name one of Barkley and Giroud in their European squad.
Had Chelsea dropped into the Europa League, joining Arsenal, then Giroud would not have been eligible.
In Emerson's case, he was not registered for Roma in their Champions League group stage squad so he is not cup-tied, meaning there is nothing to stop him being registered as one of the three players.