This is exactly it. The Pats are a zone read offense, he will know every read and what the receivers are expected to run one certain coverages. He may not have run it well, but that doesn't mean he doesn't know the offense. He would know every detail from blocking assignments, routes, audibles, and reads.
Anyone that thinks this doesn't work, go back to 1996 Jimmy Johnson cuts Kieth Byars. He was not performing as a Dolphin, the Patriot coach Bill Parcells signs him. The next game we play them we lose. In a post game interview Dan Marion admits the Pats knew everything we were going todo. Want another example, Bucs\Raiders Superbowl. The Raiders stayed with Gruedens offense, the Bucs turn in 5 picks 2 for td's and John Lynch is on the sideline laughing and saying Gannon is doing everything coach said he was going to do.
Plain and simple the Pats could change the audible signals or terms, but they can't change their offense midseason. BIG ADVANTAGE DOLPHINS! I wouldn't care if Galloway ever caught a pass!