Grant isn't worth 28 million a year, but you can make the trade for the next 1.5 years and see what happens.
Also, I think his efficiency will go up considerably when he goes back to not being the primary scorer. Obviously his scoring will go down in that scenario, but I'd expect him to go back up in the 38-40% shooting range from 3 at about 15 ppg. He doesn't do much else though, so you have to decide if a couple mid to late 1st's is worth that and at 20 million a year or so. If it is, then you make the trade if you don't give up any long term pieces.
I'd probably do this
Grant, Joseph for Richardson, Hernangomez, Fernando, 22 1st, 24 1st (lotto)
I think that is a reasonable enough trade and improves Boston enough to pay the tax and give up the picks.
I'd do that in a second.
I'd see more of a 3-way trade. Something like:
Boston gets: Jerami Grant
Atlanta gets: Josh Richardson
Detroit gets: Hernangomez, Lou Williams, Gorgui Dieng, Boston protected 1st, OKC 2022 lotto-protected 1st via Atlanta (so probably two 2nds, but OKC is only four games away from the play-in, and the West might be a tankathon the next three months, so who knows).
I wouldn't be a fan of that, personally, but if you felt the C's were committed to getting a starting 4 and moving Al to the bench, rather than using Al's salary to get a starting 4, this would make more sense than sending a useful player like Richardson to Detroit who would not use him. Atlanta seems like a decent fit for Richardson, the two outgoing salaries are expiring, and while the pick has some potential, it's likely two seconds, and the Hawks aren't short of seconds in the near future. Keeps them under the luxury tax too, unless they make the conference finals so Capela reaches his contract incentive.
Again, it wouldn't be my first choice, as I'm among those who think the Celtics most of all need an upgrade at PG (although I'd keep Smart as the starter and still move Al to the bench). I'd preserve assets/ salary flexibility for that player or a third star, and not Jerami Grant.
I should note that Jake Fischer, who's tweet/article started this thread, has a decent history of accuracy when discussing the C's. but I don't know if that's just because he quickly jumps onto what others have already posted and pretends as if it's his own, or he actually has sources.