WASHINGTON -- Washington Nationals pitcher Stephen Strasburg has been superb since his recent return from the disabled list.

Now the Nationals hope he can stick around.

Strasburg was cruising against the San Francisco Giants Saturday, but was pulled with an injury in the fourth inning of Washington's 9-3 win.

"He left the game with some tightness in his left side, so we'll continue to evaluate that," manager Matt Williams said. "We'll see how he feels tomorrow, but given his season so far, we're not going to take a chance there."

After his first pitch to first baseman Brandon Belt with two outs in the fourth, Strasburg grimaced and briefly grabbed his left side.

Williams, pitching coach Steve McCatty and the trainer went to the mound. After a lengthy conversation, Strasburg left the game and was replaced by Tanner Roark.

"It's pretty tight," Strasburg said after the game. "They said that was going to be the case. I've iced it a couple of times.

"Don't really have an explanation. Everything felt good and then just threw a pitch to (Buster) Posey (hitting in front of Belt) there. He grounded out and (I) just felt it grab."

Strasburg had allowed one hit while striking out three and walking two in his third start since missing 21 games with neck tightness. The right-hander had been sharp, winning his first two starts while allowing two runs over 12 innings.

"Part of the conversation on the mound after he exited was with (catcher) Wilson (Ramos) and (shortstop Ian Desmond) about how good he was today," Williams said. "The best they've seen him all year."

Tyler Moore had three hits and drove in four runs for Washington (45-36).

Bryce Harper, wielding a patriotic stars and stripes-themed bat, had three hits including a two-run homer, his 25th of the season.

Center fielder Michael Taylor Jr. also homered for Washington and Roark (4-3) pitched 4 1/3 innings of solid relief.

"He's versatile, he can do a lot of things," Williams said of Roark. "Today it was long relief. He stepped up."

Roark, who has worked as both a starter and reliever this season depending on Washington's needs, retired the first eight batters he faced. He gave up two runs on five hits and struck out three without a walk.

First baseman Brandon Belt had two doubles for the Giants (42-40), who have lost five straight on their current road trip.

The Giants saw a pair of scoring chances evaporate when Belt was picked off first by Ramos in the second inning, and then in the eighth when center fielder Gregor Blanco rounded third on third baseman's Matt Duffy's infield single and was tagged out by Desmond.

"This happens when you get in a streak like this. We made some mistakes that shouldn't happen on a major league field," Giants manager Bruce Bochy said.

"You're down (3-0), you got picked off first; and of course, Blanco got a little too aggressive. You want the guys to be aggressive, but you've got to play according to the scoreboard."

San Francisco starter Madison Bumgarner (8-5) was lifted after the first two Nationals reached in the sixth. He allowed a season-high six runs on eight hits.

Five pitches in, Bumgarner had given up three hits, including two home runs.

Taylor sent his first offering of the day over the wall in left, and third baseman Yunel Escobar followed with a double off the wall in center.

Harper, the right fielder, then lined a 1-1 pitch into the Nationals bullpen, picking up his 59th and 60th RBIs of the season. His previous season-high was 59 in 2012, his rookie season.

"He just got ambushed," Bochy said of Bumgarner. "They came out swinging, and it's a case where a pitcher's trying to get settled in."

It was the second time Bumgarner's been roughed up in a July 4th game at Washington, which comes with a late morning start time.

"I was definitely ready for the early game and for the heat, (but) it wasn't too hot today,"Bumgarner said. "Mentally I was locked in. Can't make any excuses about that."

NOTES: San Francisco RHP Ryan Vogelsong (6-6, 4.19 ERA) opposes RHP Jordan Zimmermann (6-5, 3.16 ERA) in Sunday night's series finale. ... Giants LF Ryan Lollis, 28, made his major league debut. ... Washington pitchers began play with a 1.93 staff ERA over their last 13 games, the lowest in the majors during that stretch. ... Nationals CF Denard Span, who has reached base safely in 23 consecutive games, was given a day off.