UEFA made a statement during the draw about the ability of on-loan goalkeeper Thibaut Courtois to take part against Chelsea. Here's the statement inf full:
"In response to media reports referring to the situation of Club Atlético de Madrid goalkeeper Thibaut Courtois, UEFA would like to reiterate its position."
"Both the UEFA Champions League and the UEFA Disciplinary Regulations contain clear provisions which strictly forbid any club to exert, or attempt to exert, any influence whatsoever over the players that another club may (or may not) field in a match."
"It follows that any provision in a private contract between clubs which might function in such a way as to influence who a club fields in a match is null, void and unenforceable so far as UEFA is concerned. Furthermore, any attempt to enforce such a provision would be a clear violation of both the UEFA Champions League and the UEFA Disciplinary Regulations and would therefore be sanctioned accordingly."
UEFA have just made things interesting, and the response of the two clubs in question will also be worth watching. It appears as if the governing body of Europe wants to see the best players from all remaining players on the pitch when available, and will do what it takes to encourage that this happens even if those players are contracted to their current club's opponent.
Naturally, when the balls were opened, Chelsea were paired with Atletico. This is likely just the beginning of a long debate on this matter, and frankly, seeing Atletico just decide to keep the keeper on the bench to ensure a good working relationship with Chelsea wouldn't be a surprising end result.