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.

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.