Whoever starts at SG for the Mavs is only a starter in name, and even so, I think Carter will be rejuvenated this year playing next to Kidd, Dirk, and with Cuban and his cryo chambers of revitalization. Marion was "done" after his stint in Toronto only to reinvent his game when coming to Dallas, same with Chandler in Charlotte.
The thing is with Carter, in Orlando and Phoenix, he was supposed to carry the load of being a number 2 scorer (next to Dwight and Nash). In Dallas, he won't have that burden / we don't need him to do that, meaning that if he ever reverts to his ball-stopping ways, RC can just pull him for Terry, Beaubois, even Jones, who I think will have a pretty decent defensive role this year. Whether Carter starts or not is irrelevant to the outlook of the team because of how small his role will be. Dirk and Terry are still the 1-2 on offense. 3rd would probably be Odom, then Marion, Carter, Beaubois, Kidd, Haywood. (and that's a contending top 8)