The principal matchup problem that the Phoenix Suns’ small, quick lineup presents its opponents is what to do with the center. Most conventional pivotmen, like the Clippers’ Chris Kaman and Dallas’s Erick Dampier, often look like St. Bernards stuck in a greyhound race when the Suns dictate the tempo. Few conventional centers have enough lateral range to stay with 6-foot-8-inch “pivotman” Boris Diaw, so Suns opponents are left to defend four on five, a death sentence against the league’s most potent offense.

But what happens when the Suns are presented with an agile, athletic big man – someone versatile enough to range all over the hardwood and guard long-range shooters, someone agile enough to defend point guard Steve Nash on the perimeter when necessary, yet quick enough to race back to the rim to either block shots or inhale rebounds? Hakeem Olajuwon and David Robinson are retired and Ben Wallace plays in the Eastern Conference,yet Dallas pivotman De-Sagana Diop did a magnificent job channeling their spirits this weekend in the Mavs’ victories in Games 2 and 3.

Diop, a 7-footer from Senegal, is fast becoming a poster boy for allowing big men a five- or six-year incubation period. A bust during his first four years in the league in Cleveland, Diop signed with Dallas during the summer and moved into the starting lineup shortly before the all-star break.His primary attributes are defense and rebounding, though he knows how to roll to the basket for easy buckets when opponents leave him uncovered. His play against Tim Duncan during the overtime period of Dallas’s Game 7 win over San Antonio in the last round is a major reason why the Mavericks are even still playing.

Diop entered Game 2 against Phoenix midway through the first period and had an immediate impact.On his second defensive sequence, he blocked a Tim Thomas layup. As the game progressed, he effectively closed off the paint – where Phoenix had so freely roamed in Game 1 – pulling down five rebounds in six minutes as the Mavericks turned an 11-5 deficit into a 22-17 lead. After Diop took the court,the Suns missed 11 of 13 shots.

Diop had a similar impact during a key stretch of Sunday night’s Game 3, starting the second half and discombobulating the Suns offense with his presence in the paint. Denied easy access to the basket, the Suns shot 1-of-6 during the first five minutes of the frame and committed three turnovers. Meanwhile, Dallas turned a 52-47 deficit into a 59-54 lead and won the quarter 25-16. By the final buzzer, the Mavs had held the Suns to just 38 second-half points,including only four fast-break points.

Phoenix’s demoralized state was best illustrated by its meltdown late in the fourth quarter of Game 3. Down 90-84, without the ball and just over a minute to play, the Suns – rather than fouling in order to get the ball back – let the Dallas run its offense, which enabled the Mavs, after yet another offensive rebound and a short jumper by guard Jason Terry, to drain a full minute and 14 seconds off the clock while nursing the six-point lead. Not even a rocketquick offensive team like the Suns can overcome an eight-point deficit in 33 seconds. The coaching staff put the team in a position from which it was impossible to recover.

The Suns may get guard Raja Bell, who injured a calf muscle in Game 1, back for tonight’s Game 4,and they need him. Without Bell, the Suns’ potent offense has been reduced to a pedestrian one, scoring 98 points on in Game 2 and 88 in Game 3. Earlier in the season, the Suns were able to win lower-scoring games when Kurt Thomas was healthy and offering a strong defensive presence inside. (Local fans may recall that the Suns beat the Nets 92-81 in November and the Knicks 85-81 the following month.) But since Thomas went out with a foot injury in late January, the Suns have had to rely solely on their offense to win; it’s failing them now.

Making matters worse for Phoenix is that Dallas still has ample room for improvement. Though he hustled and grabbed a couple of key rebounds in the fourth quarter, guard Mavs Jerry Stackhouse shot just 2-of-8 from the floor and finished with seven points in Game 3. Point guard Devin Harris also had an off night, shooting 3-of-7 for six points. And reserve swingman Marquis Daniels, who’s averaging 10 points in 28 minutes per postseason game, never made it off the bench. In other words, while the Suns’ offense will undoubtedly get a boost if Bell returns, the Mavs figure to get a boost tonight from at least one of three players who failed to contribute much scoring in their Game 3 win.

As if all that weren’t enough, the Suns face another challenge: Even if Bell returns, Phoenix is using a very short rotation, and the Mavericks are one of the best teams at drawing fouls. Dallas shot more than 28 free throws for every 100 field goal attempts this season, third best in the league. Foul trouble, particularly to the Phoenix big men – Tim Thomas, Diaw, and Marion, none of whom are even that big – will further cripple the Phoenix attack. In Game 3, for example, Diaw picked up his fourth foul early in the third quarter and wasn’t much of a factor the rest of the way.

Despite being shorthanded, the Suns have used their speed to win two sevengame series. But against a deeper, equally quick opponent in Dallas, Phoenix will be lucky to see a Game 6 in this round.

mjohnson@nysun.com