Doc Rivers is back for the final year of his contract. Paul Pierce is signed for four years and now Ray Allen has returned to the fold, agreeing to a two-year deal with a player option for the second year.

Allen’s contract is a coup for Danny Ainge and the Celtics and it’s also an indication that Allen understands the shifting landscape of the NBA. Allen wasn’t in the market for the proverbial last big contract. He needed a place where his skills could flourish, even at age 35.

He also left himself an out if the Celtics struggle, Rivers decides to leave and a new collective bargaining agreement works in his favor. The Celtics meanwhile have left open a two-year window until Kevin Garnett‘s contract expires. It really is a win-win deal for both sides.

The NBA has set the cap for next year at a tick over $58 million, about $2 million more than was expected, and lifted the moratorium on transactions. Many of the big name players have already decided (with a certain big one still to go Thursday night) and the big work for Ainge and the Celtics is done.

The starting five is under contract along with Glen Davis, first-round pick Avery Bradley, Turkish center Semih Erden and Rasheed Wallace. More on Sheed in a minute, but it’s worth pointing out that Philly legend Sonny Hill told the Globe that Wallace might be re-thinking his decision to retire.

Beyond that, Oliver Lafayette and Tony Gaffney have non-guaranteed minimum-level deals, and there’s also second-round pick Luke Harangody, who has not signed a rookie deal yet. Keeping Wallace where he is for a moment, that’s 12 players if all of the rookies make the final roster.

Now Ainge has to get creative.

Keep a few things in mind. The Celtics are over the cap and they used the bi-annual exception on Marquis Daniels last summer. That leaves the mid-level exception, valued at $5.765 million, and veteran minimum contracts.

Here are three more items of business for Ainge as he puts the finishing touches on the 2010-11 Celtics.

1. Re-sign Tony Allen

Allen has already reportedly drawn interest from the Lakers, although they used a significant chunk of their MLE on Steve Blake. Allen will probably get more interest after the big names have signed. He finally lived up to expectations on the defensive end and has turned himself into a valuable role player.

If Allen is able to continue his play during the regular season it wouldn’t be a shock to see him land on one of the NBA All-Defensive teams. That’s a big if for Allen, who has battled consistency issues throughout his career.

Still, the Celtics know what they’re getting in T.A. and would be wise to bring him back to fill the role of defensive stopper off the bench. The Celtics couldn’t give Allen away last summer, but now a two-year deal wouldn’t be out of place.

Because they have retained Allen’s Bird rights, there’s no financial limit on what the Celtics could spend to re-sign him.

2. Sign a veteran big man

Jermaine O’Neal has emerged as the likely choice for the Celtics and there’s a lot to like about him. He’s younger than Brad Miller, and while Miller had one of the worst seasons of his career, O’Neal had a bounce-back year with the Heat.

O’Neal shot 53 percent from the floor last season, a career high, and was able to knock down medium-range jumpers at a strong clip. With the way the Celtics space the floor on offense, that’s a skill they value greatly in big men.

The question is, how much of the MLE will it take to sign him?

Assuming there’s some left over, Ainge will target shooters. The C’s have been linked to Golden State’s Anthony Morrow, but he is restricted, which leaves the Warriors the right to match any offer.

They have also been linked to Rasual Butler, who might be a more realistic option. Butler is a 36 percent 3-point shooter for his career, although he struggled last season with the Clippers. But he would bring some size to the perimeter and offer a true backup small forward option.

If Ainge can get a big man and a perimeter shooter with the MLE, then the work is almost done but there is still one big card left to play.

3. What to do with Rasheed?

The Celtics think Wallace will retire, and they have used that possibility as a trade chip. In that sense, Wallace is walking cap space.

However, people close to Wallace have indicated that he may be having a change of heart. If he does return, which is his right, that would give the Celtics another big man option as they try to fill the position while Kendrick Perkins rehabs from knee surgery.

For now, the Celtics are operating as if the former is true, and they are dangling his $6.3 million contract, which would presumably come off a team’s book in a trade and a buyout.

The Herald reported interest in Phoenix’s Leandro Barbosa and Portland’s Rudy Fernandez. Both are dynamic guards who may need a change of scenery.

Barbosa is due to earn $7.1 million next season and with the emergence of Goran Dragic as Steve Nash‘s backup, Barbosa is a luxury for a cost-conscious team like the Suns.

Fernandez, meanwhile, is more of an enigma. He has shown flashes of brilliance mixed with head-scratching decisions. He is also still on his rookie contract and at 25 is far too talented to be given away for cap space.

It’s worth noting that both players are on teams that have had turnover in their front offices and in chaos there is opportunity.

Sheed’s contract remains the big wild card left to play, but for all the moves yet to come, the core of the team that went to Game 7 of the NBA finals will be back for one more run.