Interesting situation.
He was clearly called a guard when drafted, but we have to recognize that his experience is mostly at tackle. He might be most comfortable subbing at tackle, but with Light, Vollmer, and Solder, the Patriots have three solid tackles. With an aging Waters and Connolly called into service at center, Cannon's greatest value is at guard.
I can't imagine there will be a strong market for Donald Thomas if he is dropped. The Patriots can find another guard to add to the Practice Squad if they want a little more shadow depth. They do have three OL on the current PS, I believe, in Welch, McDonald, and Kopa - each of whom are taller outside OL.
Given his potential, I'd be happier seeing Cannon as the sub guard over Thomas. If there is an injury to Mankins or Waters in the next game or two, the most likely scenario is Wendell comes in at center and Connolly moves to guard. I guess I would be comfortable with Cannon as the #5 interior OL, in hopes that he can compete for a starting position in 2012.