No question, that Desmond Harrison was simply not ready to play at the NFL level. The Browns coaching staff was rather short-sighted for thinking that he was the right guy to replace the retired Joe Thomas.
His (Harrison's) numbers looked amazing from a sheer athletic point of view, but he never showed that he could at least play against the best at the FBS level, much less the NFL He's been beaten pretty badly by some mediocre defensive ends throughout the year, and when your left tackle (who is supposed to be your best pass blocker) can't do the job, it's a no-brainer that your QB is going to get pressured.
Besides, the off-the-field issues, he was simply a big fish in a little pond, where he was able to out-muscle a bunch of weaklings while playing at West Georgia. Unfortunately for him, once he got to the NFL, even the smaller defensive ends were running over him with sheer physical strength, and after he got shell shocked by that, it was easy for them to run around him.