Eh, it's not great but it's not the worst we've ever had. Marcel Jones only did 13 reps at his combine, and he was pretty solid. From other schools, Orlando Brown did 14 reps after he weighed in at 345, although it is being talked about as one of the worst recent combine performances. He was fine in college though. A couple years ago a Wisconsin lineman (Havenstein) put up 16 reps, and I didn't hear anything about Wisconsin's S&C or Havenstein's personal effort, plus he was still a second round pick. It's not a good number, but long arms are not a benefit for bench and some taller guys don't do as well as their weight would suggest.
I know Knevel wasn't great, but I'd chalk that up more to playing hurt because Cav "didn't have time to prepare backups to play." I didn't tend to notice him when he was playing healthy, which is better than I can say for some other lineman.