Complex sentence: Because Basel failed to qualify for the Champions League group stage, they dropped into the 2015--16 UEFA Europa League group stage, where were drawn into Group I alongside Fiorentina, Lech Poznań and Belenenses.
Split into simpler sentences: Because Basel failed to qualify for the Champions League group stage they dropped into the 2015--16 UEFA Europa League group stage. Basel were drawn into Group I, together with Fiorentina, Lech Poznań and Belenenses.