First, yes, Cassell endorsed Springer.
Celtics Champion Sam Cassell Endorsed Jaden Springer Trade .
But as to your edit, Springer has played 7 minutes this year and while I've not watched every minute of every game, I'm pretty sure he hasn't played any meaningful minutes. Not sure how anyone can tell how anyone looks in 7 minutes, BOS probably has a better idea from practices. And the idea is that (i) he's young, and (ii) he has already demonstrated he can play NBA on-ball defense. Plus, his salary slot could theoretically be important
But in terms of on-court, I'd much rather Walsh or even Baylor and Springer get any minutes that are available over any NBA vet including LW4. The incremental wins that LW4 might have provided isn't nearly as important as any development the Cs can get out of Walsh in particular but also Baylor and Springer.
Maybe closer to the trade deadline the Cs will figure out they won't need his salary slot and will staple a 2nd round pick to him to save the $10M or so in luxury taxes. Cs have called around gauging trade interest reports say:
Celtics gauging trade value of reserve guard (report) - masslive.com