Manchester United have completed a move for Arsenal forward Alexis Sanchez. But will he be able to play in the knockout stages of the Champions League?
So to put it simply: YES!
UEFA Champions League regulations allow clubs to register one player for the knockout rounds "who has 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."
Sanchez 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.
Had Manchester United dropped into the Europa League, joining Arsenal, then Sanchez would not have been eligible to play in that competition.