John Fox remarked that new OC Jeff Davidson isn't "new", he's just new to the staff. Apparently, he came close to finding his way on this staff before. Fox stated that Davidson "almost" joined the original staff, possibly being in the running for the OL coach or TEs coach jobs that were vacant in 2003.
However, the more likely explanation is that Fox had hoped to get Charlie Weis, his Plan A in 2002. Weis was rumored to consider leaving the Patriots to coordinate for good friend John Fox; the end sticking point was that Weis was an expensive addition to an inexpensive staff. Fox, a brand new coach, was cheaply paid (around the bottom of the league, estimated at $800k-1 million in his original contract), not in small part because the team was still paying off George Seifert and had just gotten out from under the contract of Dom Capers.
Weis, a highly esteemed coach who had just won a championship, could've tried for a head coaching job of his own. He was easily worth as much as Fox, and whether it came down to that for Weis, or came down to a cheap front office, is hard to say (and while Jim Mora Jr. was a painful problem to deal with in 2003 replacing Jack Del Rio, sources say that he strung the team along for monetary reasons as well). Coaching free agency was in full swing - coaches who had contracts coming up weren't renewing, and because of that various highly esteemed coaches were getting paid very well to go to new teams.
Either way, Weis stayed and got a sizeable raise. Fox went with "Plan B", a guy likely highly regarded and recommended by Weis, Dan Henning. Henning, a former coordinator for the Redskins, Bills, on Weis' staff as QBs coach, and Weis' successor for a year in NY, seemed to be the steady hand a new head coach needed. Had Weis come to Carolina, the fun irony is that Henning would've likely been QBs coach. Weis and Bellichick's choice for their staff in NE, Dick Rehbein, had died in the 2001 season and Henning (out of work after the failed Al Groh season) could've as easily been the QBs coach for the Patriots in 2002 if he hadn't come to Carolina.
Davidson, close to Weis and a guy who called plays for Weis for a short time in 2002, certainly would've come to Carolina as well. The idea of a Weis/Henning/Davidson staff in 2002 would've undoubtedly brought more success. Whether Davidson had any other shots at a job or not is hard to say, and he and Henning had no tie. Mike Maser and Dave Magazu did, which is probably what put them in their positions in 2003, whether Davidson had been available or been considered.