Sadly it is quite a valid concern. I just spent 5-10 minutes messing about trying to come up with a vague 12-13 man rotation, and it is darn hard.
On the one hand, he put up his best scoring and rebounding per minute season to date, and he's on a bargain contract, all the while (from what he's said) loving it in Boston.
On the other hand, Hayward is significantly better than him, and Tatum likely will be next year too. Hayward, Tatum and Brown will all be playing above 30 minutes (maybe not initially for GH), as will Kyrie and most likely Al.
A lot with Morris I think depends on whether Smart comes back or not (or Terry). If we lose one, or both, of those guys, then Brad's ability to play those 3 guard lineups he likes sometimes goes out the window basically. That would open up more minutes for Morris, but I'm not sure if he has a place here sadly. Could see him packaged with Terry & picks for something, but I don't know