Seven-time winner Milan and Arsenal are seeded in the 10-team non-champions section where potential opponents include Fenerbahce and Metalist Kharkiv — both threatened with expulsion by UEFA because club officials are implicated in match-fixing cases.
Metalist faces a UEFA disciplinary panel next Tuesday, one week before first-leg matches begin, but could appeal any ruling at the Court of Arbitration for Sport and continue to play.
Fenerbahce is already heading to CAS. The Turkish club awaits a hearing date to appeal against a two-year ban from UEFA competitions. A verdict is expected before the Aug. 29 group-stage draw in Monaco.
The other seeded non-champions are group-stage regulars Lyon, Schalke and Zenit St. Petersburg. Seedings are decided by clubs' results in UEFA competitions over the previous five seasons.
Portuguese newcomer Pacos de Ferreira joins the unseeded pool alongside Fenerbahce, debutant Metalist, 1988 European Cup winner PSV Eindhoven and Real Sociedad.
Ten national title-holders are in a separate section of the draw, includes former European champions Celtic (1967) and Steaua Bucharest (1986). They are seeded alongside Basel, Dinamo Zagreb and Viktoria Plzen.
Unseeded teams looking to break into the group-stage elite are Austria Vienna, Legia Warsaw, Ludogorets Razgrad, Maribor and Shakhter Karagandy.
Ludogorets and Shakhter, which have both advanced further than ever before in European competitions, are the true underdogs with UEFA club rankings of No. 295 and 324, respectively.
Whatever Friday's draw, Ludogorets — whose small home stadium isn't certified by UEFA for high-level matches — likely will play the first leg in Bulgaria because of a Roger Waters concert.
Only the national stadium in Sofia is suitable, and it is unavailable for the Aug. 27-28 second leg because of preparations for the former Pink Floyd star's show later that week.
"Some colossal walls are to be built and it seems that we use the national stadium as a concert hall instead of its actual purpose," Ludogorets owner Kiril Domuschiev told Bulgarian television channel bTV.
Kazakh champion Shakhter scored an upset victory over BATE Borisov — which last season beat eventual winner Bayern Munich in a group match — and followed up by eliminating Skenderbeu of Albania this week.
Each playoff round team collects 2.1 million euros ($2.8 million) prize money, and the 10 winners join 22 elite teams which qualified directly for the lucrative group stage. The 32 group teams will share around 900 million euros ($1.2 billion) on offer for the rest of the competition.
Complications could pile up for UEFA if Fenerbahce and Metalist win their playoff matches on merit but are then removed for legal reasons. Their defeated third qualifying round opponents this week — Salzburg and PAOK Thessaloniki — could also seek to be reinstated.
UEFA's Champions League rules require clubs not to have been involved in fixing matches since April 2007. Fenerbahce's case dates back to 2011 and Metalist's in 2008. Both clubs deny wrongdoing.