This is certainly very bad. You can't gloss over that. But I wouldn't writer the season off yet. We just now need even more good fortune than we did before. And we needed a lot before.
As Who said, if Marquis some stays healthy, he can fill most of the void that Green has left. When healthy Quis is a starter-quality wing who does a lot of things well. There's no reason he can't play 30 mpg and eat up most of the backup minutes at the 2/3.
I'd also strongly consider bringing back Posey. Yes, he's fallen off a bit, but I think coming back here could revitalize his career much like Branch's career was revitalized by coming back to Brady. I said when Posey left that he really only would succeed in a system of big time talents that would deflect attention from him to allow him to do the little things. He's obviously not a player who you can lean on offensively. He clearly didn't have that in Indy, but he could have that here.
I'd also strongly consider toying with lineups with JJJ at the 3. He can hit the 20 footer, is superathletic, and is quick. And at the 3 is slight frame wouldn't be an issue. We could also post him on smaller 3s. Do we want him guarding elite 3s? Maybe not, but he should be able to handle backup 3s in the NBA.