Oxlade-Chamberlain had to be replaced early on in the Champions League semi-final first leg after sustaining a knee injury while challenging Aleksandar Kolarov.
Speaking post-match at Anfield, Klopp told BT Sport: “Oxlade, it’s probably a really bad injury.
“If we can say that already before the scan, that’s never good news so that means for us it’s a massive blow, of course.
“We have still a few very important games to go and the squad doesn’t get bigger in the moment, so we obviously need to be creative in the next few games.”