The “Big Three” is now officially the “Big Four” in Boston.

What was labeled the “Big Three,” in Kevin Garnett, Paul Pierce and Ray Allen officially has a fourth member in Rajon Rondo.

The Celtics point guard has matured in front of our very own eyes during the 2010 NBA playoffs.

The 24-year-old Rondo will be the x-factor in the NBA Finals against the Los Angeles Lakers. As Rondo goes, so does the Boston Celtics. His 16.7 points in the playoffs is third best on the Celtics behind Paul Pierce and Ray Allen and his 10.0 assists is tops for all players. If the Elias Sports Bureau kept track of the number of times a player out hustled their opponent, Rondo would be on the top of that list as well.

With a favorable matchup against the aging Derek Fisher, Rajon Rondo could be a household name and elevate himself as one of the league’s top guards if the Celtics win their 18th NBA Championship.

Coming into the playoffs, Rondo didn’t crack anyone’s list (outside of Boston, of course) of the top 5 point guards in the NBA.

Now?

Not only would Rondo crack the top 5 list, but you can make a strong argument that he’s the best point guard in the NBA behind Chris Paul and Deron Williams.

Rondo has improved leaps and bounds since he was the 21st overall pick from the University of Kentucky in 2006. During his rookie season, he split time off the bench with Delonte West and Sebastian Telfair, who the University of Louisville favored over Rondo in 2004. Rondo moved into the starting lineup in 2007-08 and hasn’t looked back. His 794 assists this past regular season set a new Boston single-season franchise record, breaking Bob Cousy’s 50-year mark.

“I’m just trying to become a better leader,” Rondo said. “My numbers are what they are, but that’s because we still go through the Big Three — they’re the main focal point. That’s why I’m able to get so many open looks and be so aggressive — because the defenses are trying to take something away from Paul, Kevin or Ray. I’m just able to capitalize on some of the mistakes they make.”

The last time Rondo was on the national stage during the 2008 NBA Finals, he averaged 9.3 points, 6.6 assists, and 3.8 rebounds against the Lakers. Outside of a 16-assist outburst in Game 2 and a 21-point performance in the decisive Game 6, Rondo was a non-factor in the series.

“He’s a point guard now that runs our team and has complete control of our team,” head coach Doc Rivers said. “When we won it (in 2008), he was still learning how to be a point guard. He was still trying to figure out how to help a team just win. Now we rely on him to win.”

Prior to the 2010 NBA Playoffs, the knock on Rondo was the lack of a jump shot. Let’s be honest, the guy playing pickup games at the YMCA probably has a better jumper than Rondo — that’s how bad it’s been at times.

But Rondo showed signs of improvement during the H.O.R.S.E competition at the 2010 NBA All-Star Game, trading 3-pointers at the top of the key with eventual winner Kevin Durant. Granted, Rondo was shooting without a hand in his face, but nobody expected the career 24.4 percent three-point shooter to have a shot at winning.

His hot hand has carried over to the playoffs. In 17 games, Rondo is shooting 38.9 percent (7-for-18) from beyond the arc. Hardly anything spectacular but it forces teams to think twice before leaving him wide open.

“Probably the curse for Rondo was I was a point guard,” said Rivers. “And that may be good, and probably bad at times for him as well. But what’s gratifying to me is just seeing him work. You work hard enough and the results eventually will come for you. He’s done that. That’s not for me or anybody else. He did that, and that’s why he’s playing well.”

Fisher will have his hands full trying to contain Rondo. The 35-year-old point guard has been tested in the playoffs against Oklahoma City’s Russell Westbrook, the Utah Jazz’s Deron Williams and the Phoenix Suns’ Steve Nash. All three scored at will against Fisher during their respective series but they ultimately weren’t a deciding factor in the outcome.

Fisher’s best bet at trying to slow down Rondo is hoping he’s not 100 percent. Rondo suffered muscle spasms during Game 4 of the Eastern Conference Finals against the Orlando Magic. During practice on Monday, Rondo participated in 75 to 80 according to Rivers. If healthy, Fisher and the Lakers are in for a long series.

“I’m very confident in what I do,” Rondo said. “Doc and I are becoming better at getting on the same page, and I just have to continue maturing.”