The Danish outfit finished down in 18th place during the league phase after winning only two games and had to contest a dramatic play-off clash against Heidenheim, whom Chelsea beat earlier in the competition, losing 2-1 at home last week before bouncing back to win the second leg 3-1 in Germany on Thursday night as Mexico winger Rodrigo Huescas popped up with the crucial goal in extra time.
Betis could have faced Chelsea in the last 16 but now take on Portuguese side Vitoria SC instead, while Fiorentina have a potentially tough test against Panathinaikos, another team beaten by Chelsea during the league phase.
If Chelsea get past Copenhagen as expected, then they will face either Norwegian team Molde or Polish side Legia Warsaw in the quarter-finals, with the second leg away from home.
Elsewhere in the last 16, Polish side Jagiellonia Bialystok meet Cercle Brugge of Belgium and Slovenian side Celje have been paired with Swiss team FC Lugano.
Win that too and a semi-final looms against either Cypriot side Pafos, Djurgarden of Sweden, Bosnian team Borac or Austrian outfit Rapid Vienna, with the second leg again away.
