This team is too far from contending to use a guy like Deng right now. If they were one piece away, he'd be great. Right now he'd waste away a bit on the Celtics while taking in a large chunk of salary and driving many Celtics fans nuts. Essentially, a better Jeff Green.
There are a lot of great free agent centers (Hibbert, Chandler, Gasol) available in 2015. If we could lock up Deng and Rondo for a few years, draft a high caliber 2 guard, and make do with the Sullinger/Olynyk combination at PF, why couldn't we be back in contention in 2015?
Where's the money coming from for one of those free agent centers?
Figure we are at $50M with Rondo maxed, $12.5 in Luol Deng, and $12M in first rounders. Tax line is probs what, $80M at that point? I think there's space for another max deal and a few more pieces. Wallace comes off the books the following year, and then we can re-sign Olynyk/Sully.
If you're signing a free agent, the applicable line is the salary cap, not the luxury tax. An aggressive estimate would put that closer to $65 million.
The other option will be for the Celtics to work out a S&T for said center using Wallace's (finally) expiring contract as the primary salary equalizer, but that will require some significant compensation to the player's original team, or a third team willing to take Wallace, on top of the compensation to the team losing the center -- assuming said team is willing to participate in a sign-and-trade at all. It's not impossible, but getting that center in 2015 via free agency is not going to be a slam dunk.
Mind you, I'm still pro signing Deng for 3-4 years at $11-12 million per year. I like Jeff and all, but I think Deng is an upgrade and worth it (when the alternative is 1-2 more years of Green at $9.2 million per year). Of course, if we draft Wiggins or Parker, then that's a different story.