Tony Gwynn Jr. is stepping up big time for Dodgers

When talking about Tony Gwynn Jr. this week, Dodgers Manager Don Mattingly had no idea how prophetic his words would later sound.

“Tony’s that backup guy,” Mattingly said, “but he ends up playing a ton.”

Gwynn could play a ton more, depending on the severity of the injury that forced center fielder Matt Kemp out of Wednesday’s game in the second inning. Gwynn replaced Kemp, who was only a day removed from being activated from the 15-day disabled list.

During the 15 days Kemp was sidelined because of a strained hamstring, Gwynn was forced into a prominent role.

Gwynn, considered one of baseball’s top defensive outfielders, took Kemp’s spot. And with the speedy but inexperienced Dee Gordon struggling offensively, Gwynn also moved into the leadoff spot into the Dodgers’ order.

Gwynn went into Wednesday with five steals, second-most on the team behind Gordon’s 14.

“He’s really swung the bat well,” Mattingly said.

A career .249 hitter through Tuesday, Gwynn hit .304 in the 14 games he played while Kemp was out.

Gwynn has particularly excelled this season with men on base.

With runners on base, Gwynn was batting .381 (16 for 42) through Tuesday. He was hitting even better with men in scoring position: .480 (12 for 25).

Gwynn also played a more significant role than expected last season. He was signed in the winter leading up to the 2011 season to be the Dodgers’ fifth outfielder. He ended up playing in 136 games and starting 63 times.

Booing Braun

Want to boo Ryan Braun? Go ahead, Milwaukee Brewers Manager Ron Roenicke said.

“I think sometimes it works in our favor,” Roenicke said.

Roenicke believes Braun is motivated by the jeers, which are becoming increasingly frequent.

Of Dodger Stadium in particular, Roenicke said, “It certainly motivates him here.”

The 2011 National League most valuable player, Braun grew up in nearby Granada Hills.

Roenicke said the boos directed at Braun at Dodger Stadium on Tuesday were about as intense as they have been anywhere this season. Braun was two for three with a home run and a walk Tuesday.

“I don’t understand it,” Roenicke said. “If the guy didn’t do anything wrong, I don’t understand it.”

Roenicke was referring to the drug test Braun failed during the playoffs last year. Braun’s 50-game suspension was later overturned.

Braun’s appeal was based on how the sample was handled. The positive test itself was never challenged and Braun has never explained it.

“For me, he has,” Roenicke said.

But Roenicke conceded he could understand how some fans think Braun hasn’t done that.

If Dodgers fans are upset that Braun edged out Kemp for the MVP award, Roenicke said they shouldn’t be.

“Kemp is a great player,” Roenicke said. “If he keeps doing what he’s doing, he’s going to win some MVP awards.”

Short hops

Juan Uribe, who is on the disabled list because of an injured wrist, took batting practice. … With Gwynn starting on the bench, second baseman Elian Herrera led off for the Dodgers. James Loney batted second.

dylan.hernandez@latimes.com