WASHINGTON - Rookie right-hander Stephen Strasburg went on the 15-day disabled list Thursday with inflammation in his throwing shoulder, the latest indication of how careful the Washington Nationals are being with their prized pitcher.

"They're taking every precaution with me, obviously," Strasburg said.

It's the first trip to the DL for Strasburg, who was the No. 1 overall pick in the 2009 amateur draft and signed a record $15.1 million contract.

"If he's 90 percent healthy, we shut him down," Washington manager Jim Riggleman said. "He's got to be 100 percent."

Riggleman said the team expects Strasburg to pitch again for them this season. It's possible he could begin his road back by playing catch Saturday.

The move to the DL is retroactive to July 22, a day after Strasburg made his last appearance. He was scratched minutes before he was to make his 10th major league start Tuesday, because his right shoulder felt stiff while he was warming up in the bullpen.

An MRI exam taken Tuesday showed Strasburg's shoulder is "totally clean," general manager Mike Rizzo said.

"It's feeling great already. Keep making big strides. So just going to keep getting better, keep getting stronger, and when the time comes, I'm going to be ready to go," Strasburg said.

He's been shut down since Tuesday, resting his shoulder and getting treatment.

Strasburg is 5-2 with a 2.32 ERA, 75 strikeouts and 15 walks in 54 1-3 innings since making his debut for the Nationals on June 8, when he struck out 14 in a victory over Pittsburgh.

The Nationals have said they would end Strasburg's season when he reaches 160 innings between the majors and minors, even if that cap were to come in late August or early September. So far in 2010, his count is at 109 2-3, including the minors, which essentially matches his workload in his final college season at San Diego State: 109 innings.

Riggleman will need to find someone to take Strasburg's turn in the rotation Sunday against the Philadelphia Phillies. Strasburg said he will head out on the road with the Nationals when they travel to play the Diamondbacks and Dodgers from Aug. 2-8.

"We're really looking at only at about eight or nine more days until he'll be able to pitch again," Riggleman said. "We'll probably go another few days after that."

By sending Strasburg to the DL, the Nationals made room on the roster to activate left-hander Scott Olsen, out since May with a shoulder injury. Olsen started Washington's game against the Atlanta Braves on Thursday.