I'm surprised that Memphis is actually this willing to bet on a major roster change this much.
Anyway, Pierce for Gay straight up is probably a good deal for the Celtics. But it also means "Jeff Green is our future" talk gets in the trash bin, and with his trade value taking a dip, there is no way we are dumping Green at somewhere else for something good.
Not sure why the Grizzlies would want to replace Gay with Pierce, though. To be fair, Pierce is still the better player out of two at this late age, but he's probably going to retire if he gets traded midway through this campaign. Basically, Grizzlies would be rolling the dice with a roster including Pierce for a single off-season and see what happens.