I've been a fan of Nerlens since before he was drafted. If this idea was proposed a couple months ago, nobody on this forum would say "no"... and I'd probably laugh off the idea as absurd. But things change.
Philly has attempted to play Nerlens out of position at PF and it's a disaster. He excels at protecting the rim. They have him running around the perimeter. I had Nerlens as the runner-up to Wiggins in Rookie of the Year last year. He came back from his injury and just got better as the season progressed. Over the second half he put up 13.1 points, 10 rebounds, 1.9 assists, 2.3 blocks, 2.1 steals with 49% shooting and 66% from the line. Additionally, Philly had a top 12 defense thanks to him and the top defense in the league in minutes he was on the court. He put up historic defensive numbers for a 20 year old rookie. Flash forward to this season, he's struggled with injuries and seems completely inable to co-exist with Okafor. His numbers have taken a nose-dive to 10.3 points, 8 rebounds, 1.2 assists, 1 block and 1.5 steals with 43% shooting and 58% from the line. He's still only 21 years old. Philly needs to shake it up at some point. His stock isn't getting any lower than it is right now.
On the flip side, if you believe Bill Simmons, Boston was ready to cut Sully this Summer had he not gotten into shape.

I actually wasn't as down on Sully as everyone else. He's always been able to contribute with minutes. And he doesn't turn 24 years old for a few months. He's had probably his best season to date... putting up averages of 10.2 points, 9.9 rebounds, 2.5 assists, 1 block and 1 steal with 43% shooting and 64% from the line in just 26 minutes. His per-36 numbers are pretty impressive: 14.2 points, 13.8 rebounds, 3.5 assists, 1.3 blocks, 1.3 steals ... Still, I have plenty of concerns about him. I feel he has a limited ceiling and questionable attitude.
Why for Boston: Many reasons. For one, Sully is a restricted free agent this summer so we may be stuck paying him big money next year... cutting into our flexibility to add talent via free agency. Second, I just don't think Sully has anywhere near the ceiling of Noel. Boston could slot Noel in at his natural center position. He has the potential to be a dominant defensive force and the idea of him, Bradley and Smart together long term is a freaky proposition. Olynyk can get the bump to starting PF. I've long been a believer in Olynyk's shooting ability. With minutes, he's performed (and has actually been more or less even with Sully over the past few years in per minute production). I also feel like Noel and Olynyk would be a good fit next to each other. Noel isn't an offensive contributor, but Olynyk's shooting makes up for it. Two other factors to consider... #1 - I don't see Amir as a long-term solution here. He's here in part, because we gave him a weird 12 million unguaranteed contract that can make him exceptionally tradeable should a star become available. I don't anticipate Amir to be here making 12 million next season. #2 - If we hang onto that Brooklyn pick... our pipe dream is Ben Simmons. Ben Simmons is a PF. At some point, we need to clear out space at that position anyways... so might as well get rid of Sully before we're stuck paying him.
Why for Philly: That's less clear. They'd be buying high on Sully and selling low on Noel. In theory, I imagine Sully would fit better next to Okafor. Sully can hit some shots and even stretch the floor a tiny bit (though he's only shooting 30% from three). Sully is a natural power forward so it might make some sense. He's certainly a better offensive player than Noel at this point. If I'm Philly, I look for more than Sully... and probably look for better options from other teams... but a trade idea is a trade idea, so I'm throwin it out there. If Colangelo is really in charge (doubt it) and really wants to make immediate steps towards putting something resembling a team out there this year (doubt it), moving Noel for Sully might have some sense to it.