Looking at the replays it seems Kabasele was sliding beside the net, but his cleats caught the net and it kinda spun him a bit into the post. Seemed like he got bent over backwards on the post a bit.
Stayed down for several minutes, finally sat up, walked around, and then came back on the pitch. But basically right away went down again and went off. Spent the next 20 or so minutes on the bench before being wheeled off on a stretcher from there with an oxygen mask on.
On the bright side, the medical staff didn't seem to be in a hurry with the stretcher and all seemed to be walking at a normal pace, so hopefully it's not too serious.
He did seem to be flexing his right arm when he came back on the pitch the first time, and then held it at his side when he went down again which makes you think some sort of nerve pinch or similar and is always scary when a back gets bent like that.