PHOENIX — Third baseman Casey McGehee comes to San Francisco with a “4” practically stitched to his forehead, the number of home runs he hit in 616 at-bats for the Marlins last season.

Ron Roenicke, who managed the Brewers when McGehee hit 13 homers in what Roenicke called an “off year,” said he believes McGehee’s power ceiling is higher than what he showed in Miami, where he played in a ballpark more cavernous than AT&T Park.

“There’s more there,” Roenicke said during a Major League Baseball media day Monday. “When you want to be a good hitter and you know what you have to do to drive in runs, sometimes the ballpark makes a big difference in what your power numbers end up being.

“He’s got power. If he wants to try to hit more home runs, maybe he doesn’t drive in as many. He’s got to play his game.”

Milwaukee traded McGehee to the Pirates after the 2011 season. By 2013, McGehee was playing in Japan.

“I like what he’s done with his swing,” Roenicke said. “I like what I saw last year compared to the year I had him. There’s less movement in his swing now than when he was with us. He had a big rock when he was with us. He’s quieted down his swing some.”

Bumgarner revisted: Kansas City manager Ned Yost is convinced that his team was beaten by one man in the World Series: Madison Bumgarner.

“Without Madison Bumgarner, we’d be World Series champions right now,” Yost said. “We’re not. What he did was amazing.

“When I go back and try to re-evaluate our playoff run, I look at it that we lost four games during the entire playoffs, and three of the games were the direct result from Madison Bumgarner’s performances. As hard as it is to swallow, it was a historic performance. You can’t take that away from him.”

Bumgarner spent part of his Monday posing with a bull, in a Scottsdale Stadium parking lot, for a Giants Magazine photo.

General manager Brian Sabean did not see the Bumgarner photo, but said, “I’m waiting for him to body-slam Aoki. He’s getting practice throwing down bags of cement.”

When the Giants signed Nori Aoki, the new left fielder joked that he wanted to wrestle Bumgarner.

Henry Schulman is a San Francisco Chronicle staff writer. E-mail: hschulman@sfchronicle.com Twitter: @hankschulman