PHOENIX -- After the Nationals' 8-4 loss to the D-backs on Thursday night, Washington placed center fielder Nyjer Morgan (right hip soreness) on the 15-day disabled list, retroactive to Wednesday.

"I hope it's just two weeks," said manager Jim Riggleman, after reviewing Wednesday morning's MRI results with his leadoff man and the club's medical staff.

Washington also announced transferring pitcher J.D. Martin (bulging disk in back) to the 60-day DL, presumably to open a 40-man roster spot for the player who will fill Morgan's cleats.

Riggleman said that player would be "a bat off the bench" and wouldn't specify if an outfielder or infielder would receive the call-up. He all but ruled out promoting a third catcher -- Triple-A Syracuse's hot-hitting Wilson Ramos.

Morgan, who has felt lingering tightness his hip flexor since attempting to track down a fly ball in a July 21 game against the Reds, said after Thursday's defeat that he "feels like he can play," and that he didn't understand why he was being deactivated.

Morgan will stay with the club during its trip this weekend to Los Angeles, but, while his presence can only aid his teammates, the Nats' outfield didn't miss a beat without him in the final two games of its series in Arizona.

Roger Bernadina, who will bat leadoff and man center field during Morgan's hiatus, went 4-for-8 with a home run and a stolen base in his stead. Willie Harris will back up Bernadina in center.

In addition, Mike Morse, who has shifted into Bernadina's right-field spot, went 2-for-3 and smacked his seventh home run on Wednesday before an 0-for-4 day at the plate on Thursday.

"Having good at-bats, seeing the ball, working deep in the count -- usually when I find myself doing that, I have good results," said Morse, who is batting .324 (25-for-77) when he starts this season. "We got some good outfielders on the bench and playing. Until [Morgan] comes back, I'll do whatever I can."

Strasburg throws sim game; to start Tuesday

PHOENIX -- Stephen Strasburg declared himself ready to return to the Nationals' rotation on Thursday following his 43-pitch simulated game.

Strasburg, who will return from the 15-day disabled list on Tuesday to start against the Marlins in Washington, twirled his entire repertoire on Chase Field's mound against reserves Willie Harris and Justin Maxwell.

"My mechanics were a little out of sync," he said, "but that's to be expected not having a batter stand in there for close to two weeks.

"It feels really, really good right now. I was a little anxious out there. I flew open a little bit on a couple, but there [were] a lot of good pitches in there."

Strasburg will throw in the bullpen before the Nationals play the Dodgers in Los Angeles on Saturday, affording him one more opportunity to shake off the rust.

"I'm almost encouraged that he's frustrated with his mechanics," manager Jim Riggleman said, "because that means he knows he has to continue to work, that he's a work-in-progress."

Strasburg said the soreness in his right (throwing) shoulder subsided two days following his expected start on July 27, which was called off when he struggled to loosen up in the bullpen.

"I knew it was just a matter of time," he said. "I just needed a little blow, and I needed to get my strength back. It happens. I'm going into a new area in the season I have never pitched at before, so you got to be smart about it."

Strasburg, who has thrown 109 2/3 innings between the Minors and Majors this season, is 5-2 with a 2.32 ERA through nine starts for the Nationals.

Marquis set to return Sunday vs. Dodgers

PHOENIX -- The Nationals announced Thursday that Jason Marquis will return from the 15-day disabled list in time to start on Sunday in Los Angeles.

A corresponding roster move will be required to activate Marquis.

Marquis, who had bone chips in his elbow, made two rehab starts for Triple-A Syracuse, most recently allowing two runs on three hits over six innings on Monday. The veteran is signed for $7.5 million in 2011.

"He wants to contribute," Nats manager Jim Riggleman said. "If he's the real Jason Marquis, the guy who is sinking the ball and getting ground balls and attacking hitters, he can really help us and be a part of our future."

Riggleman added that lefty Scott Olsen, who was slated to start Sunday, remains in the club's rotation plans. Olsen would pitch as early as Wednesday. Stephen Strasburg, also coming off the 15-day DL, will pitch Tuesday.

Worth noting

FOXSports.com reported Thursday that Nationals first baseman Adam Dunn, who hit a pair of home runs in Wednesday's 7-2 win over the D-backs, has been claimed off revocable waivers. A Washington official declined to comment. The Nats can now negotiate an unlikely trade with the anonymous club that claimed Dunn. Nineteen of Dunn's 28 dingers this season are solo shots. ... Catcher Ivan Rodriguez received a normal day off on Thursday. Rodriguez had started the Nats' previous three games.