As late as May 27, the notion that Nate Robinson would return to the Celtics seemed far-fetched. Danny Ainge’s trade deadline acquisition had fallen so far out of the rotation he was spending more time with Marquis Daniels on the bench than with Paul Pierce or Ray Allen on the court.

But then on the night of May 28, Rajon Rondo went down hard on the floor in Game 6 of the conference finals against the Magic and all of Celtics fandom held its breath. Enter Nate Robinson.

In a little over eight minutes Robinson scored 13 points and the Celtics were on their way to averting disaster and en route to the NBA finals. The X-factor had finally arrived.

“I told him I loved him, and I told him at some point this was going to happen,” Doc Rivers said after Game 6. “It was all up to him to stay engaged, and he did. I get no credit for this. Nate Robinson stayed focused in 30-straight whatever games without playing. To me that’s more important than anything he’s done tonight.”

Once Robinson delivered on the court, Rivers kept him in the lineup and he provided valuable backup assistance to Rondo against the Lakers. Still, while Robinson had finally found a niche with the 2010 Celtics, there was some debate as to whether he would still fit with the team going forward.

Then Tony Allen spurned the Celtics to sign with the Grizzlies and re-signing Robinson became a higher priority. To be sure Robinson and T.A. have very little in common as players, but as Ainge looks to fill out the bench with only the veteran minimum to offer, quality players at any position have become a premium.

Now Robinson is back in the fold and that’s good news for the Celtics. Here’s what he has to offer.

1. A capable backup for Rondo

The Celtics got by with Eddie House playing minutes behind Rondo, but that was never an ideal situation. Ainge brought in Sam Cassell and then Stephon Marbury, but neither player worked out in that role.

Robinson is not a pure point guard, but he can handle the ball and he provides instant offense off the bench, which can be a game-changer. The key is knowing when to play the Nate card and when to pull it off the table and Rivers showed a deft touch with giving him enough opportunities to create, but not so much as to allow him to shoot his way out of the game.

Robinson also committed to playing fullcourt pressure defense on opposing point guards and that became a key to the Celtics defensive scheme. If he can do that, and make a few shots, for 15-20 minutes a night then the backup point guard situation is solved.

He also provides insurance if anything were to happen to Rondo. Few players attack the rim, and take as many hard hits as Rondo. With Robinson on board, there is a clear alternative should Rondo get banged up during the season, which is almost a given.

2. Second-unit offense

As it stands, the Celtics bench consists of Glen Davis, Robinson and a bunch of potential. We don’t know how much players like Avery Bradley, Semih Erden and Luke Harangody are going to be able to contribute next season.

Robinson’s return takes the pressure off Bradley as he can now develop in a timely fashion without the burden of expectation that would accompany a rotation player.

He also lessens slightly the need to find a shooter on the wing. Make no mistake, the Celtics want shooters at as many positions as possible, but his offense allows Ainge the flexibility to bring in a more defensive-minded player if he goes that route. The pickings are slim in terms of free agent wings and no one currently out there is an ideal fit.

3. The X-factor

In the brief time that Robinson was with the team he instantly endeared himself to the fans with his on-court personality. He is, in a word, fearless. Think the thought of going up against the budding super-team in Miami is going to faze him? No way.

He is also one of the best pure athletes in the game today and he brings an energy and a swagger to the Celtics that would have been absent without him. Robinson is often described as a boom or bust player, but in the right role his talents are a welcome addition to a team with a clear goal.

The 2010-11 Celtics are not a finished product yet, but they look a lot more complete today than they did yesterday.