Effin Mo Williams.

What the Boston Celtics need Rajon Rondo to do isn’t easy.  Not in the least.

He needs to seize the torch from the Big Three as the go-to Celtic, and he needs to do it in the playoffs.  Not only that, but he has to do it against the Eastern Conference’s best team and the galaxy’s best player.  Because, while Rondo has been the baddest Celtic all season long (bad meaning good), he has also shown a disappointing tendency to fade down the stretch of games and defer to his older, more famous and — suddenly — less talented teammates. Rondo, as Mo Williams says, is the Celtics’ engine, but does too much sputtering when the game is on the line.

Just like he has all season, Rondo looked brilliant for long stretches of Game One but weakened down the stretch.  He didn’t quite disappear, but his dropoff in aggression was more than noticeable.  His star teammates, especially Kevin Garnett and Paul Pierce, got more touches in the fourth quarter but did less with them and Boston lost a very winnable game.

“I should have called maybe more pick-and-rolls for myself,” Rondo said, “but at the same time I didn’t want to keep on calling plays for myself.”

But if the Celtics are to reach their potential — if they are to derail Mr. MVP, his supporting cast and that goddamn elbow we keep hearing so much about — then Rajon Rondo is going to have to call his own number more often.  I’m not trying to say he needs to be selfish — far from it.  Rather, he needs to be aggressive.  There is a huge difference between being selfish and aggressive.  Steve Nash is one of the league’s most aggressive players — always probing, always penetrating — but also one of its most unselfish.  Nash is always attacking, but always trying to improve his teammates.  That type of unselfish aggression is exactly what the Celtics need out of Rondo.  And especially down the stretch.

The fourth quarter is where so many Celtics games this season have gone to die.  Three times this season, including Game One, the Celtics have allowed the Cavaliers big fourth-quarter runs.  Two resulted in Celtics defeat, and the other an all-too-close win after a 20-plus point lead evaporated.

The Celtics are statistically a very good first-half team but only average in the second half of games, and Rondo leads the second-half fizzle.  I couldn’t find the statistics but Rondo seems to always play second fiddle to the Big Three come crunch time, even when he has huge games until that point.  In fact, ever since Rondo missed two pivotal free throws after having his number called in the waning moments of a Dec. 27 bout with the Los Angeles Clippers (a game Baron Davis would win with a buzzer-beating jumper only 1.5 seconds after Rondo two missed free throws could have given Boston a lead), Rondo seems to have been almost non-existent in the final minutes of games.

Now, part of Rondo’s late-game disappearance is that neither he nor Doc Rivers have faith in Rondo’s jump-shot and scoring ability.  Understandable.  Rondo can’t be relied on to score at the end of games because, well, he’s not a natural scorer.  Even when compiling 19 points in Saturday’s first half, Rondo wasn’t being “a scorer;” he just took what was open and dished out 8 assists to go with all them points.  While he can occasionally put up big numbers in the points column, the Celtics can’t rely on Rondo to score down the stretch because he struggles to put the ball in the hole against half-court defenses.  In the final minutes, when play inevitably slows down and open space becomes limited or non-existent, Rondo has a tougher time putting the biscuit in the basket than players with more diversified offensive games.

But that doesn’t mean he’s useless. Rondo can still get past his defender and make a play.  Whether that ends in Rondo scoring or an open shot for somebody else, Rondo’s penetration generally ends with good results.  Especially against the Cavs, when neither Mo Williams or Anthony Parker could stay in front of Rondo even if Rondo had anvils attached to each of his shoes, he can get to the lane at will.  He can create offense as easily as you can say “fast as lightning,” and has a distinct speed advantage that should allow him to get into the lane even in halfcourt sets.

It’s not as easy as Rondo knowing he can get into the lane, though, or knowing he can create plays.  Rondo is one of the most cocksure players in the league, a guy who already considers himself the game’s top lead guard and knows what he can do.  The problem instead lies in the fact that there’s also the not-so-little task of stealing crunch-time glory from the more established Big Three. As far as stars go, Paul Pierce, Kevin Garnett and Ray Allen are rather ego-less but, even for such selfless studs, it’s tough to see a 22-year old steal crunch-time touches and take control of the team.  It’s bad enough that the Big Three has become shells of themselves — now, you expect them to step aside and allow a 22-year old with a hockey player’s jumpshot lead the way?  I’m sure they’re willing to do whatever it takes to win, but to do so would be to acknowledge their own slipping games and, for players as competive as the Big Three, such an admission is tough.

That’s the conundrum for this Celtics squad: Rondo is the best player, but he’s not the one in control.  He still concedes touches, and especially down the stretch, to the Big Three though the Big Three has shown signs of no longer being able to carry the team in the clutch.

“It’s a tough one for Rondo at times, because he’s really conscious of trying to get Paul and Ray (the ball),” said Doc Rivers.

But for the Celtics to play their best and finally close out the Cavs with an impressive second half, Rondo’s going to have to be conscious of getting someone else the ball.  That someone else should be clear:

Himself.