When the defending champion San Antonio Spurs sprinted out of the gate, winning 17 of their first 20 games, it seemed that they were on a holy mission to repeat as titlists. Back-to-back rings were about the only thing that the team had not accomplished during their run of four titles since 1999. But then the going got tough. Since that opening burst, they have descended to an un-Spur-like 11–13 and are now losers of three in a row, going into tonight’s game at Phoenix, the third game of a nine-game road trip.

The Spurs mediocre play of late has raised some eyebrows, but it seems that they do this every year: go through a period of middling ball before finishing the season like gangbusters. But this season may be different. The Spurs are a different team when one of their big three — pivotman Tim Duncan, swingman Manu Ginobili, or point guard Tony Parker — has been out of the lineup. And now comes news that Parker will be sidelined indefinitely with bone spurs in his left heel. This isn’t some midseason malaise; it’s a real problem for San Antonio.

Throughout the decade the Spurs have been known as a tightly knit team. They play rock solid defense and resourceful offense. Although Duncan is one of the best players of all time, neither he nor his teammates are the sort to garner enough street cred to be videogame stars. The Spurs won because their stars played like stars and their role players were among the best in the game. They were businesslike and ruthlessly efficient. However, this year appears to be different. The Spurs’ role players, almost all of whom are somewhat long in the tooth, seem to be slipping fast.

Guard Michael Finley, who turns 35 in five weeks, is a career 44.4% shooter, but so far this season he’s chucking and ducking at 38.7%. Matt Bonner, a career 47.2% shooter, is managing only 41.2% this season. Francisco Elson, a career 49.1% shooter, is down to 42.6% this season. And Robert Horry won’t be getting many chances to live up to his nickname of “Big Shot Bob.” The career 42.6% marksman is bricking them at a putrid 18.2% rate. The lone role player who has countered this trend, 37-year-old Brent Barry, is out for three weeks with a foot injury. Small wonder teams are double teaming Duncan and getting away with it more.

The Spurs aren’t the same team when one of their big three is out. They are 1–4 without Parker, and they went 7–6 during a monthlong stretch from December 5 to January 4 when Parker, Ginobili, and Duncan were on the court together for only two minutes.

Duncan’s absence can be offset somewhat by the play of center Fabricio Oberto and Ginobili’s absences were covered well by Barry, but without Parker, the Spurs fall into a deep hole. For years, the Spurs attempted to groom Yugoslavian guard Beno Udrih as a backup for Parker, but his style of the play and Spurs system proved a poor fit. Udrih, in his first NBA season out of Texas, is averaging 13.2 points a game on 45.1% shooting for the Sacramento Kings. Meanwhile, the Spurs have gone with Jacque Vaughn, a former Net who averages only 9.1 points per 36 minutes. Contrast that with 19.9 from Parker.

Right now, Parker, Duncan, and Ginobili account for 58.4 of the 96.3 points per game that the Spurs average; that’s 60.6%. That is the most dependent that the Spurs have been on their three stars for their offense. Last season, it was only 55.9%. In 2005-06, the big trio accounted for 55% of the San Antonio offense. They scored 54.9% of the points in the title run of 2004–05 and even less, 54.4%, in 2003–04.

With Parker out indefinitely and the role players not coming through, the Spurs are closing in on a deal with Damon Stoudamire, who completed a buyout of his contract with Memphis and became a free agent last night, in order to bolster their bench with a veteran. According to a report by ESPN, Stoudamire’s agent informed the Spurs that he intends to sign this week. The Spurs are a stat savvy team and realize that Stoudamire will help them while Parker is on the shelf, and he should contribute from the bench when Mr. Longoria returns.

While these struggles spell trouble for San Antonio, they shouldn’t spell doom — at least not until May. Although the Western Conference is very competitive, the Spurs early season flurry wasn’t a fluke. Parker is sitting now to be fresh for the stretch run. The champs may have to enter the playoffs as a lower seed than usual, and it would be a mistake to take them lightly.

mjohnson@nysun.com