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Brandon Morrow's abbreviated start could turn out to be an extended problem for the Toronto Blue Jays.

Morrow left after just nine pitches with a strained muscle in his left side and the Blue Jays lost 6-3 to the Washington Nationals on Monday, their fourth defeat in five games.

Morrow had won three of four starts coming in, a stretch in which he'd also thrown two shutouts, but was gone before an out was recorded in this one.

After giving up a leadoff double to Steve Lombardozzi, Morrow (7-4) motioned for the trainer after missing low on a 1-1 pitch to Bryce Harper, the second hitter of the game.

"I felt it on the first pitch to Harper," Morrow said. "I felt like a stabbing in my side when I threw it."

Following a brief discussion involving manager John Farrell, Morrow walked off the field and down the steps to the clubhouse.

Morrow continued to feel tightness in his lower rib cage after coming out of the game and will be reevaluated Tuesday. Farrell said the injury was "probably substantial."

"He was in considerable pain," Farrell said. "There will be further imaging, further testing in the morning."

Neither Morrow nor Farrell would speculate on how long the right-hander might be sidelined. Morrow called the injury "very frustrating."

"Hopefully it will just be a best-case scenario," Morrow said. "We still don't know what that is."

Washington manager Davey Johnson said the early change put obvious strain on Toronto's relief corps.

"I know what it does to your bullpen so I was sympathizing with (Farrell)," Johnson said. "Not a whole lot, but I was sympathizing with him."

Chad Beck came on to replace Morrow and immediately gave up an RBI single to Harper.

"That's never happened to me before," Harper said of facing two pitchers in the same at-bat. "It was kind of weird to go back up there and get into the zone again and get comfortable."

Harper went to third when Ryan Zimmerman reached on a fielding error by first baseman David Cooper, then scored on Michael Morse's sacrifice fly.

Toronto's Colby Rasmus cut the deficit in half with a solo drive to right in the bottom half, his ninth, but Adam LaRoche responded with a two-run shot to right in the third, his 11th.

Washington tacked on one more in the seventh on an RBI single by Ian Desmond, and another in the eighth on Rick Ankiel's drive to left, his fourth. The homer came off Blue Jays right-hander Francisco Cordero.

Harper had three hits in his return to the starting lineup and right-hander Edwin Jackson pitched eight strong innings to win consecutive starts for the first time this season.

Jackson (3-3) improved to 4-0 in his past seven starts against Toronto dating to July 30, 2008. The right-hander, who had gone eight starts without a win before beating the New York Mets in his last outing, allowed two runs and three hits. He walked three and struck out three.

Left-hander Sean Burnett came on after Edwin Encarnacion's leadoff double in the ninth and gave up a two-run homer to pinch-hitter Yan Gomes, his third, before finishing off the win.

The Nationals pounded out 14 hits, two shy of their season high for a nine inning game, and improved to 5-2 in interleague play this season. Washington has won seven of nine overall.

Three of Washington's hits came from Harper, who was held out of the starting lineup Sunday because of back pain. Harper, who went 3 for 4, also walked and scored twice.

NOTES: Nationals OF Roger Bernadina strained his right hamstring while advancing from first to third on a wild pitch in the fourth. He was initially replaced by Xavier Nady, but Ankiel took over defensively in the seventh. Bernadina will be re-evaluated Tuesday. ... Beck was optioned to Triple-A Las Vegas following the game, with LHP Evan Crawford and LHP Aaron Laffey recalled from Triple-A. Toronto will make a second roster move Tuesday to make room on the roster, with Morrow likely headed to the DL. ... Washington RHP Drew Storen (right elbow) threw off the mound Monday for the second time since undergoing surgery. Storen is not expected to return until after the All Star break. ... Nationals RHP Ryan Mattheus (left foot) will make a second rehab appearance with Class-A Potomac on Tuesday, and is expected to work one inning or more. ... The Blue Jays signed 22 draft picks Monday, including first round selection OF D.J. Davis.