I (and many others) have said this before, but the main issue with Noel is that this is the 3rd year of his rookie deal. He has two more years left, this year and next. After that he'll be a restricted free agent.
The question then becomes: Do you think Noel might be worth 25% of the salary cap when his rookie deal is through? Because that would be the max for a player of his experience, and it might be what you'd need to spend to retain him, or risk losing him for nothing.
Given all this, I wouldn't give up much for him, especially because you just signed a guy who's a prototypical new-age center (Horford).
Given the current salary landscape, I wouldn't worry too much.
Pretty much every free agent with starter (or even fringe starter) talent is out there raking in max / borderline-max deals. When you have 20+ teams with fat wallets, all fighting over the same 5-10 free agents, then you end up with a bidding way. Overpaying is inevitable.
At least in Noel's case (unlike with Westbrook / Griffin) he'd be restricted, which means the Celtics have flexibility. If we want to keep him, we have the right to match all offers. If we don't want to, then we have the power to pressure potential suitors into a S&T. Either way we don't need to worry about the risk of him walking away for nothing. That justifies giving up reasonable assets if need be.
Noel is likely never going to be a big star, but he's already one of the better defensive centers in the NBA. If he came to Boston there's a chance me might be the best defensive player on the team from day one, and that's no mean feat given the defensive talent already on this roster.
Then there is the future - at 22 years of age with his length, athleticim and natural defensive instincts...he has the potential to be a Dwight Howard / DeAndre Jordan / Kevin Garnett caliber defensive player. IMHO a player with that type of defensive upside is worth taking a gambling on with a "post-rookie" max deal.
Remember we would get two years of Noel before he'd even reach restricted FA, so that gives us two seasons to track his progress and see what happens. After two seasons if he doesn't show substantial improvement, you can always cut your losses. But if he starts flashing serious DPOTY potential, then you lock him up quick smart.
This would basically be like a corporate lease. You get to drive the car for three years, and if you REALLY like it, you have the option to buy it at the end. It's a pretty good position to be in to be honest.