A serious knee injury diminished his athleticism, which was off the charts in his first few years. It didn't diminish his toughness one iota.
If I recall correctly, the Eagles traded two 1st round draft choices to get him.
He ran into a contract issue with the Bengals. While still a Bengal, he signs a personal service contract with the owner of what would be the Virginia Ambassadors in the World Football League. That was signed in 1974, but his Bengals' contract ran through the 1975 season. The Bengals sued Bergey for breach of contract, the WFL, and its franchises. The Bengals lost that case.
However, Bergey never joined the WFL, as in July of '74, he was traded by the Bengals to the Eagles, and signed a 5-year contract with the Eagles. The Eagles, who had a history in the early and mid-70s of trading away high draft choices, sent the Bengals their 1977 1st round pick, and their 1978 1st round and 2nd round picks, for Bergey.