Rajon Rondo wasn’t supposed to play against the Pacers on Monday night. About an hour before the game, Celtics coach Doc Rivers told reporters in Indiana that Rondo would sit out his second straight game to rest his injured finger. Then, Rondo convinced the coach that he was fine ... and when trainer Eddie Lacerte agreed, Rondo was back in the lineup.

Rondo’s first quarter was a whir of strong drives to the basket, perfect finishes and transition offense. In other words, he was everything that he hasn’t been over the last dozen games. Rondo made his first five shots and finished with 22 points on 9-for-13 shooting, eight assists and one turnover. He also made all four of his free throws.

You can dwell on the way the Celtics eventually lost the game, 107-100, to the Pacers (click here for a full recap) and Rondo’s part in the collective defensive breakdown in the fourth quarter, but the fact that Rondo was finally Rondo again is more important for the Celtics than the final score.

Losing a winnable game hurts, no question. They blew a golden opportunity to pick up a game on the Bulls who lost their second straight game and also fell into a tie with Miami for second place in the East. That’s a major problem as the Celtics try to salvage something out of the final nine games of the regular season.

But unless Rondo found his game, seeding wouldn’t matter anyway. No matter how many O’Neals come back or whether Nenad Krstic regains his confidence, the Celtics are going nowhere unless all of their big four are playing at a high level and Rondo’s demise has been the principal reason behind their slide.

Heading into Monday’s game, Rondo had been shooting 33 percent over his last 10 outings and his percentage at the rim has dropped to a meager 45 percent — down 20 points from his season-long average. He’s been dishing out just 2.7 assists at the rim during that span, down from 6.7 in December when he was playing his best. It’s not an exaggeration to say this has been the worst slump of his career. 

His struggles have had a cumulative effect on the rest of the team as they have had to work harder for shots and their shooting percentages have tumbled. All of that has been the single biggest reason for their 5-7 record since March 6. The Celtics defense, while not as consistently dominant as it’s been in the past, has been more than good enough to win most nights. They haven’t won because they don’t score enough points and that’s been largely because of Rondo.

Monday’s game was a step in the right direction. Now he has to continue playing at that level for all four quarters if the Celtics are going to get back on track.

Here are three more points:

AN O’NEAL WOULD HAVE HELPED

Jermaine O’Neal joined the team after working out on his own — and on his own dime — in Chicago and by all accounts he looks healthy and ready to go. The plan is for him to practice with the team on Wednesday and then try to play Thursday against the Spurs.

His presence is most definitely needed. Foul trouble knocked out both Kevin Garnett and Krstic for long stretches of the game and left Glen Davis in the untenable position of trying to stop Indiana’s 7-foor-2 center Roy Hibbert. Davis played as well as he could have with 20 points in 36 minutes of action, but he couldn’t stop Hibbert, who went 9-for-10 in the first half en route to a 26-point performance.

When Garnett picked up his second foul with five minutes left in the first quarter, the Celtics had a commanding 22-12 lead. When he returned, they were losing 37-35.

The Celtics aren’t asking for much from O’Neal. If he can give them 15 minutes of defense and rebounding a night, they will be a much better team for it. While Davis has handled himself well in the pivot (for the most part), he’d be a much more effective player getting spot minutes at the position where he can be a change-of-pace player instead of being asked to hold down the middle for long stretches.

Additionally, O’Neal’s presence will take some of the heat off Krstic, who was more aggressive going to the basket when he wasn’t sitting on the bench with foul trouble. An offense-for-defense rotation of Krstic and O’Neal would be enough to answer their problems in the post, assuming Krstic can overcome his slump and regain his confidence and O’Neal can provide anything more than a tease.

Those are not unreasonable demands of both players, but it’s up to O’Neal to begin providing some return on the large investment the team made in him over the summer.

THIN DEPTH EXPOSED

While the O’Neals recover from their injuries, there are really only eight active players that Rivers trusts, and the bench has been reduced to Davis, Jeff Green and Delonte West. Troy Murphy has provided nothing. Sasha Pavlovic has done little in his short minutes and Avery Bradley simply isn’t ready.

Outside of Davis, who actually played starter’s minutes against the Pacers because of the foul trouble, Green and West provided little relief for the Celtics.

West played 30 minutes on Sunday and was not sharp in the quick turnaround, going 1-for-5 in 16 minutes. Green rarely asserted himself on offense, taking just three shots in 28 minutes — although he did get to the line at the start of the fourth quarter, which kept the Celtics afloat for a time.

But the reserve players couldn’t keep the momentum going from the starter’s hot start and didn’t provide a lift in the bridge minutes between the end of the third and the start of the fourth quarters. The bench didn’t lose the game, but they didn’t do much to help win it either.

BACK-TO-BACKS AND A FOURTH QUARTER COLLAPSE

The Pacers shot 67 percent in the fourth quarter and outscored the Celtics, 26-15. The Celtics shot 28 percent and missed six free throws. They also threw the ball away with some of the worst turnovers this side of a CYO game.

These were all classic fatigue signs of a team playing the second game of a back-to-back on the road. They’re also not surprising to anyone who’s watched this team over the season as the Celtics have been downright awful on the second night of back-to-backs.

Their inability to deal with the demands of the schedule have resulted in some of their worst losses of the season and effectively taken them out of contention for the top spot in the East.

The Celtics are now tied with the Heat for second place in the East and the difference between second and third includes a first-round matchup with the confident Sixers or a reeling Knicks team, as well as homecourt in the second round.

How many wins in the last nine games will it take? Six at the minimum, but more likely seven. That’s why losing this game stings. Just like the Clippers game and the Bobcats game and the Grizzlies game and the Nets game. The Celtics have dug this hole and while there have been glimmers of optimism here and there, they still have not found the light.