Mohamed Salah's late strike secured a 1-0 victory over Chelsea at St Jakob Park on Tuesday night.
Basel forced Peter Cech in to a early save in the 14th minute, after Fabian Frei found himself in acres of space but his low effort was deflected out by Cech.
Peter Cech was called in to action in the 25th minute, as Basel's Mohammed Salah tested the Czech with a shot from inside the area, but Chelsea's number one was equal to the task
Basel were at it again in the 36 minute, Cesar Aziplicueta misjudged his header which allowed Salah through on goal but the defender managed to redeem himself and stole the ball back for the visitors.
Basel took the lead in the 89th minute through Mohammed Salah, the striker managed to outpace Branislav Ivanovic and poked the ball past the onrushing Cech and secured all three points for the host.
Mourinho's men failed to produce a single effort on target during the 90 minutes.