Makes some sense to cover Celtic-AC Milan as the main commentary match: Man Utd's game is live on TV in any home with any sort of TV, Arsenal's game is live in any home with any form of digital TV (I'm pretty sure), but Celtic's game is only on a pay channel. Also, a match against AC Milan has far more *feeling* about it, in the context of the European Cup/Champions' League throughout history, than one against Lille or PSV Eindhoven, for all that the latter did once win it. If it was Man Utd or Arsenal against AC Milan, I'm sure that would be live even if it was on ITV1 and Celtic's game was only on Sky.
Who remembers the mid-late 80s days when Scottish clubs' European ties got more live radio coverage than before or (generally) since, entirely because of the post-Heysel ban on English clubs? Mike Ingham used to commentate on those games (often featuring Rangers when Souness had lured a load of English players who wanted to play in the European club competitions without having to move to the continent) for Radio 2 MW alongside the underused Roddy Forsyth, who should really be commentating on Tuesday IMHO.