LAS VEGAS -- Houston Rockets rookie guard Amen Thompson has a Grade 2 ankle sprain and will be out 3 to 4 weeks, a source told Andscape.

The No. 4 overall pick in last month's draft, Thompson will be in a boot for a week and is expected to miss the rest of summer league.

Thompson underwent an MRI on Saturday after suffering the injury in his debut Friday against Scoot Henderson and the Portland Trail Blazers. He left with a left ankle injury after taking an awkward fall when he attempted to block a shot under the basket in the fourth quarter.

Thompson finished the game with 16 points, 4 rebounds, 5 assists, 3 steals and 4 blocks in 28 minutes. He was slated to play his twin brother, Ausar, and the Detroit Pistons on Sunday before the injury.