Lee is the only one that is definitely not coming back after this season. He's a good player and I like what he's been doing, but he's a rental that really should finish his career being a 3rd big on a contender as his career dwindles down. He shouldn't be the 3rd big on a young team that's on the rise. Nothing against the C's but they aren't currently a contender. I foresee Lee being that 3rd big on a team like say the Clippers or the Rockets next year.
Johnson and Jerebko are the maybes depending on if the Celtics do decide to trade for a star. I don't see them staying past their 2nd year if the Celtics pick up their contracts just because of who else the Celtics have and what they do with their picks if they keep them.
The ones that are hard to determine are Zeller and Sullinger. Sully's played great this year so far but you have to wonder if a. Johnson and Lee are pushing him to play his best and b. He's just playing for a new contract. The Celtics leverage is that he will be a restricted FA this year and they can decide whether or not he's worth it. I want to see what Sully can show us throughout the whole season. I think we all know that Sully could a great piece going forward, but we don't know if he will be.
The difference between Zeller and Sully is that IMO, Sully has higher potential than Zeller does. Zeller is a nice player to have around. He is ideally a 3rd/4th big on a contending team, but with who they have right now, do the Celtics really need to have a guy who's ceiling is being a rotation big? Nope, we need that cornerstone big. Sullinger has the upshot of being a David West/Luis Scola type player, which is fine, but that means Sully, if he hits his potential, is a number 2 big on a team. Zeller is passable, but not great. I don't see Zeller on the team past this year.
The only guy I 100% believe will be on the team going forward is Olynyk. His inconsistency is frustrating, but he makes the Celtics better when he's on the court (highest plus/minus on the roster I believe). Plain and simple. He looks like he has improved different areas of his game and I think he is the first big we want off the bench when the Celtics return to contention. He's a good passer, an improved defender, and hopefully when he gets it together, should be a floor spacer.
The X factor is Jordan Mickey and how high the Celtics are on the kid. They gave him a rich deal for a second-rounder, and I see the guy more like a Leon Powe/Taj Gibson kind of player, which is perfect for a second-rounder if you ask me. If he turns out to be better than expected, that could affect every big's spot on this roster.