Numbers don't lie: Celtics need KG

Just in case last season was not evidence enough, the past 10 games have only reinforced what was already obvious: The Celtics need Kevin Garnett. Badly.

Last season with Garnett in the fold, the Celtics were 44-13 and seemingly destined to repeat. Without him, they stumbled to an 18-7 record, 25-14 if you include the postseason.

This season, the Celtics are once again winning about 75 percent of the games in which Garnett has played (22-7). Without him, they are reeling, with losses in six of their past 10.

So yes, the Celtics are better off with the future Hall of Famer on the court. That goes without saying. His expected return Friday is a welcome momentum shift for a Celtics team that hasn't used its regular starting lineup since Dec. 20.

But why? What is it about Garnett that's so valuable? Why, despite having added Rasheed Wallace, are the Celtics unable to survive without him?

One word: defense.

Garnett no longer needs to be the offensive force who averaged 20-plus points per game for nine straight seasons. He hasn't scored more than 26 points in almost two years. Wallace and even Glen Davis are capable of putting up the 15 ppg that Garnett now averages. That's not replacing Garnett entirely, but with Ray Allen and Paul Pierce in the fold, the Celtics can afford to give up some offense.

However, perhaps no player in the league matches Garnett's overall defensive impact. He is to the defense what a great point guard is to the offense. He makes his teammates better, covers up their mistakes and changes the tenor of the game. The numbers tell the story.

With Garnett in the lineup, the Celtics are allowing 91.8 ppg and holding opponents to a 43.8 field goal percentage. Without him, they allow 98.9 ppg and a 46.2 field goal percentage. In 11 games without Garnett, opposing teams' primary power forwards have averaged 20.8 ppg. That includes 37 points from Dirk Nowitzki, and two games in which Chris Bosh put up 25 points and 31 points, respectively.

It's even more telling to look at when Garnett is on the court -- not just if he appeared in the game.

According to the Elias Sports Bureau, the Celtics are averaging 101.9 points per 48 minutes with Garnett on the court, while their opponents put up 91.2 points per 48 minutes. In other words, the Celtics outscore their opponents by an average of 10.7 points per 48 minutes with him playing.

But when Garnett is not playing -- both when he has missed games and when he is on the bench -- the Celtics put up 97.1 points per 48 minutes compared with their opponents' 95.0 -- just a 2.1-point advantage.

Last season, the difference was remarkably similar, if even more stark. The Celtics were plus-12.8 points per 48 minutes with Garnett on the court and plus-2.2 when he was providing intimidating stares on the bench or watching at home.

While eye-popping, even those stats don't tell the whole story. After all, the numbers without Garnett include garbage time, as well as games when other key figures were absent.

Indeed, to fully understand his defensive impact, consider how the Celtics fare with an otherwise constant lineup. When Garnett is joined by Boston's other four regular starters, the Celtics are allowing a mere 90.5 points per 48 minutes. Their plus-173 is the best plus-minus of any lineup in the league, just as it was in each of the past two seasons.

But when Rondo, Allen, Pierce and Perkins join up with Garnett's replacements, it gets ugly on the defensive end. With Wallace, they are allowing what averages out to 100.3 points per 48 minutes. Replace him with Brian Scalabrine, and that goes up to 101.6.

Clearly, the Celtics' regulars need Garnett out there to play the kind of shutdown defense that brought banner No. 17. Take a look at the chart with the plus-minus per 48 minutes for the starting four with Garnett, and without him. All four are above plus-10 with Garnett on the court. Without him, those numbers plummet.

Rajon Rondo, plus-11.8 points per 48 minutes with Garnett, is just plus-0.7 without him. Even more startling is Kendrick Perkins. A plus-13.3 with Garnett, he is actually a negative without him.

The same pattern held true last season. The Perkins-Garnett combo was plus-12.2 points per 48 minutes. Without Garnett, Perkins was just plus-0.6 points per 48.

That's not to say Perkins has played poorly or is ineffective without Garnett. On the contrary, he's had a breakout stretch in the past 10 games, averaging 13.7 ppg and 9.0 rpg. But it's clear the Celtics are never better defensively than when Perkins and Garnett are on the floor together.

With the likelihood of a Garnett return Friday, the Celtics hope to get back to their winning ways. If the past is any indication, Garnett -- and his defense -- will help Boston do just that.

Jeremy Lundblad is a researcher with ESPN Stats & Information. He provides statistical analysis for ESPNBoston.com.