SAN ANTONIO — Tim Duncan is on crutches and the San Antonio Spurs don’t know when he’ll be back.

Duncan passed an MRI test Tuesday with no signs of structural damage in his sprained left ankle. The Spurs won’t know how long the 34-year-old might be out until later this week, but Duncan will miss at least the next three games as the NBA’s winningest team tries to clinch the West’s No. 1 seed.

“It structurally looks good,” general manager R.C. Buford said Tuesday. “We will have a better idea of what the timeline is in the next 48 hours.”

Duncan sprained the ankle Monday night in a win over Golden State.

Four minutes had barely passed in the game when Duncan, after making a short jumper, landed awkwardly on his left foot and collapsed beneath the basket.

The two-time MVP lay clutching the foot for more than a minute in the scariest moment for the Spurs all season.

San Antonio has reclaimed its place atop the NBA behind the durability of Duncan, Tony Parker and Manu Ginobili after injuries derailed the four-time champions the last few years.

Duncan will not travel with the Spurs on a three-game road trip that begins Wednesday in Denver. The Spurs (57-13) have a seven-game lead over the Los Angeles Lakers for the top seed in the West with 12 games remaining.

Only one of San Antonio’s remaining dozen opponents — Sacramento — is out of the playoff chase.

Buford said Duncan’s ankle is between a grade 1 and 2 sprain. He said team doctors are waiting for the swelling to subside to see how the ankle responds to early treatment.

“There’s a lot of frustration,” Buford said. “But I think he’s pretty tough and you guys saw him on the court.”

Buford and the rest of San Antonio’s front office left their box at the AT&T Center and went to the locker room after Duncan limped off the court Monday night, steadying himself with the help of trainers.

Duncan is averaging career lows (13.3 points, 9 rebounds) and the Spurs no longer revolve their offense around the two-time MVP. But the Spurs are quick to say Duncan is still what makes them tick.

After stumbling into the playoffs as the No. 7 seed a year ago, San Antonio is on pace to surpass its franchise-record 63 wins set in 2005-06.

If there’s an upside to the injury, it’s that rookie Tiago Splitter may use this opportunity to build experience and stamina for the playoffs. Splitter was the marquee signing for the Spurs last summer: a 7-footer and the reigning MVP of the Spanish League, and at 26 years old, primed to immediately contribute.