"I envision him as a catcher," Weiss said. "But I also envision him doing this from time to time to be able to keep that bat in the middle of the lineup. So it's a nice option to have.
"As opposed to catching, it's a considerable break when you get him out from behind the plate. He wants to play, he doesn't ever want to come out of the lineup and as a catcher you're not going to play 150 or 160 games back there. So this is a way to keep him involved."
After finishing 3-for-4 with an RBI in Tuesday's loss, Rosario is hitting .317 with five homers and 19 RBIs in August. His 71 RBIs lead all big league catchers, and his 20 homers are tied for best in the Majors.
Before starting at first Aug. 11, Rosario had just one career start at first base.
The Giants have adopted a similar strategy with 2012 National League MVP Buster Posey, who has 16 starts at first base this year.
Ian McCue is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.