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8/24/2014 7:51 P.M. ET

Cabrera day to day with rib-cage tightness

WASHINGTON -- Nationals second baseman Asdrubal Cabrera left Sunday's 14-6 win over the Giants before the eighth inning because of tightness in the right side of his rib cage. He suffered the injury while connecting on a ground ball to shortstop in the bottom of the seventh, and Danny Espinosa replaced him for the final two frames.

Manager Matt Williams said Cabrera is day to day moving forward, as the Nationals embark on a nine-game road trip that includes series at the Mariners and Dodgers. Cabrera said he doesn't expect to miss any games, but it depends on how he feels Monday morning before the Nationals open a series in Philadelphia.

"We didn't want to take any chances in that situation," Williams said after Sunday's game. "We'll have to see how Asdrubal is when he wakes up tomorrow when we get him to the ballpark. Just a little bit of tightness, so we want to be cautious in that regard."

If the injury does force Cabrera to miss time, Espinosa will likely receive starts at second base over the next week, as could utility man Kevin Frandsen.

Espinosa was in a starting role at second base earlier this season when Ryan Zimmerman was sidelined for 44 games with a right thumb fracture, but he struggled mightily from the plate and saw his season average dip close to .200. So when Zimmerman went down again in late July with a Grade 3 right hamstring strain, the Nationals opted to trade for the veteran Cabrera instead of returning Espinosa to the starting lineup.

Even so, during August, the switch-hitting Espinosa has gotten spot starts against lefties because of his solid splits from the right side of the plate -- he's hitting .298 as a righty and .183 as a lefty.

Now, there's a chance Espinosa will have to resume full-time starting duties, which he said he's continued to prepare for on a daily basis by upping the velocity on the pitching machine in the cage, among another things.

"The role this year has been when guys get hurt, being in there every day," Espinosa said. "And if not, just have to keep your head up and keep grinding and keep doing what you can to maintain your swing and maintain game speed."

In his first at-bat Sunday after replacing Cabrera, which came from the right side of the plate, Espinosa scooted a ground-ball single up the middle. Perhaps more importantly, in his second at-bat, the second baseman crushed a left-handed two-run home run over the right-center-field fence.

It marked Espinosa's first extra-base hit from the left side of the plate since July 21.

"I've done it before so I know I can do it." Espinosa said of his lefty batting. "I know it's there."

Harper continues power surge with homer vs. Giants

WASHINGTON -- In the bottom of the eighth inning Sunday afternoon at Nationals Park, Giants reliever Juan Gutierrez delivered a first-pitch four-seam fastball to Bryce Harper. The ball was letter-high and just a shade inside. Harper turned his hips, connected with the barrel of the bat and sent a line drive screaming down the right-field line.

As Harper watched from the batter's box, the ball clanked off the yellow foul pole for a home run, giving the Nationals a six-run lead in an eventual 14-6 victory in the series finale against San Francisco.

It was the 21-year-old's fourth home run in his past 17 games. Before that stretch, Harper had three home runs all season, though he missed 57 games with a torn left thumb ligament earlier in the year.

The recent power surge from Harper hasn't been a result of increased focus on long balls, according to manager Matt Williams. The first-year skipper said Harper's opposite-field double off left-hander Jeremy Affedlt in the sixth was much more indicative of the reason for Harper's recent success.

"He's hit a lot of balls back through the middle in recent days, which tells me that he's seeing it and staying back," Williams said. "It's funny how those come when you're hitting the ball back through the middle. ... If he hits a ball through the middle consistently, he'll get pitches to hit where he can pull it and be able to hit a ball over the fence."

Thornton thriving with Nats by throwing strikes

WASHINGTON -- Nationals reliever Matt Thornton entered in the top of the ninth inning Saturday night against the Giants with a comfortable four-run lead. But back-to-back singles from Pablo Sandoval and Michael Morse to open the frame prompted pitching coach Steve McCatty to visit the veteran lefty on the mound.

After a short discussion, Thornton refocused and needed just four more pitches to end the inning. He forced Joe Panik into a 6-3 double play and Andrew Susac into a flyout to center, clinching a 6-2 victory for the Nationals.

Thornton's ability to respond Saturday illustrated the 37-year-old's enhanced approach in 2014.

In 2013 with the White Sox and Red Sox, Thornton surrendered 15 walks over 43 1/3 innings. But more importantly, eight of those 15 walks ended up scoring later in the frame -- a number that impelled Thornton to question the way he was attacking hitters.

Quite simply, he needed to throw more strikes and use his high-90s fastballs to go right after hitters. And this season, the left-hander has issued only six walks in 31 innings, all of which came in a Yankees uniform before the Nationals traded for him via waivers earlier this month.

"Last year, I learned a pretty valuable lesson reiterating how bad walks hurt you," Thornton said. "One of my goals this year is to throw strikes no matter what the situation was to make them put the ball in play."

As a result, Thornton has thrived during his brief stint in Washington. Entering Sunday's action, the left-hander had retired 20 of 25 batters he'd faced as a National.

On top of his altered mindset, Thornton said manager Matt Williams has also aided his success significantly. During his time in Chicago, Boston and New York, Thornton was used primarily as a lefty specialist, largely because of his weak splits against right-handed batters. But in Washington, Williams has used Thornton less as a situational reliever and more as a full-inning one.

Only nine of Thornton's 46 appearances for the Yankees this season lasted one inning or more. By contrast, six of his eight appearances for the Nationals entering Sunday lasted a full inning.

"Any time you're a competitive person ... and people start telling you that you can't face a certain batter or you're on your way out, it kind of motivates you and pushes you to go out and execute," Thornton said, "and continue to show that you can throw a full inning."

Worth noting

• Nate McLouth was in the Nationals clubhouse Sunday wearing a large black sling with padding on his right arm. The left-handed slugger underwent season-ending shoulder surgery earlier this week to repair a torn labrum.

Daniel Popper is an associate reporter for MLB.com. He also can be found on Twitter @danielrpopper. This story was not subject to the approval of Major League Baseball or its clubs.