Actually, he hasn't. In his second season, he was beginning to emerge in training camp as a possible starter, which is good for a fourth round pick, then he suffered the devastating knee injury. He was really behind the eight ball in training camp this year because it generally takes two years to fully recover from that injury. He looked very stiff out on the field, and he didn't have enough game experience to compensate for his restricted movement.
That's what's sad about this. He never really had a chance.
If Bigby beats out Manuel, it's not out of the question that Manuel might be cut, because I don't think he would take the demotion well and Thompson would rather keep a younger player with more upside. What needs to happen is for Culver or Peprah to make a case for being on the roster. Culver has had a very quiet training camp, after showing some promise last year, although personally I thought he was a bit overrated. Peprah got an interception against Seattle, so maybe he'll get a chance to prove himself with the second string this week.