In July 2013, Celtic faced IF Elfsborg in the qualifying rounds. Celtic player Mo Bangura was on loan with Elfsborg. When the issue of his availability came up, UEFA had the following to say (emphasis mine):

"As we, in line with the FIFA regulations on status and transfer of players, consider a loan to be a transfer, we do not have any regulations regarding players not playing against their parent club whilst on loan to another club. If a player is duly registered on the player list of IF Elfsborg then he can play against any team that Elfsborg are drawn against." "Any agreement between the two clubs that this player wouldn’t play against Celtic should Elfsborg be drawn against them is purely between the clubs. UEFA would not have any involvement or consideration of this agreement, it would be neither endorsed or enforceable by UEFA."

Make of this what you will, but it doesn't seem as if Europe's governing body are being particularly consistent in their stance. Is that really too much to ask for?