See.... the way I see it there is are at least two different things here:
Play by Play Commentator: the commentator that gives voice cues to how the game is going, time left, score, etc this would be our Mike Gorman.
Color Commentator: the one actually making comments about the game, favors the team they are commentating on enough to say negative things about people opposing said team; players, coaches or officials. On a good day(Home Games) Tommy is this person.
Now, Donny Marshal is... not a good fit? He would be a great Play by Play guy, but we already have one and Mr. Gorman is just awesome at it. Donny Marshal isn't a big enough Celtics fan to be a color commentator. Generally this role should be filled by an ex player/staff member as they will have a personal history with the team beyond just watching them play from the outside(or as an opponent). Donny never played for the Celtics and doesn't know either the past players or even current players well enough to fill in Tommy's away game role.
I hate to sit and think about how there are so many fans, that haven't even played the sport on even a collegiate level, that know more about the game as it relates to the Boston Celtics than Donny does.
This is just a quick write up of why I am not a Donny Marshal fan. He can have a broadcasting job, he is good enough for it, but he certainly is not fit for taking over road games in Tommy's place.