It's hard to know what to expect from Simmons if he sees the court in this series. The reports have seemed at odds with each other, with some of them expressing confidence that he'll be ready in a week and others that say he was just cleared for contact and 5-on-5 work. I guess it's not impossible for both of those things to be true, but it probably means that Simmons' workload will be limited by his conditioning.
Do they think he's going to jump in on the deep end and shut down Tatum after having missed an entire season and changed teams? A healthy, in-shape Simmons has been a good answer to Tatum in the past. Not sure how well a rusty Simmons with an iffy back and in a new system can be expected to guard him though. Also, it's worth noting that Tatum has improved in the time that Simmons has been out, too. What used to work on him defensively might not necessarily work on him now.
It's possible that Simmons glues that Nets team together, solving their biggest defensive issues and giving them another competent decision-maker on offense. It's also possible that he's just not ready for playoff basketball and the Celtics play him off the court.