Mercer has seen considerable action at third base in the Minors, including 10 appearances there last season with Triple-A Indianapolis. But his big league time at the position was limited to one inning in a late-2012 game.
"I'll just try to have fun with it," Mercer said. "It's more about reaction time there. And you know that if you just knock a ball down, you have more time to make the throw, because it didn't take as long to get to you."
In the rare times when manager Clint Hurdle has given Pedro Alvarez a day off, the pinch third basemen had been Brandon Inge (no longer with the club), Josh Harrison (who started in right field Friday) and Russell Martin (who remained behind the plate to catch Charlie Morton ).
In Martin's two starts at third base, the Bucs had Michael McKenry to catch. Rookie Tony Sanchez is the only other receiver on the current roster.
"I felt we were best served leaving Russ where he is now," Hurdle said. "And I've seen Mercer do a lot of drill work at third, and during Spring Training. We felt comfortable moving him there."
Alvarez had faced Bumgarner only twice, and both at-bats resulted in strikeouts. At any rate, Hurdle was looking for the proper time to give him a timeout after he'd started 70 of the last 71 games.
"It's more about maintenance," Hurdle said of Alvarez's absence from the lineup. "He's been very active, on both sides of the ball, and he hasn't eased up on any of the pregame work."
With a two-run double in Thursday night's 10-5 victory, Alvarez matched his career high of 85 RBIs, first reached last season.
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer. This story was not subject to the approval of Major League Baseball or its clubs.