Matt Downs, who was in the starting lineup on Friday night against Giants left-hander Madison Bumgarner, will start against lefties and Scott Moore against righties, manager Brad Mills said.
"Anytime you get a right-handed hitter and a left-handed hitter in a platoon-type situation, you are going to get the best from both guys," Mills said.
Although neither player is a prototypical first baseman, Mills said he thinks both can handle the job, offensively and defensively.
"Scotty Moore has played there quite a bit at Oklahoma City, and from what I understand he did quite well," Mills said. "Downs played quite a few times the last few years and he does a good job."
Mills said the reason he used Chris Johnson at the position for the first few games after the Lee trade to the Marlins is that he wanted newly acquired Matt Dominguez to stay at third. Dominguez was sent down on Sunday, though, leaving Johnson to go back to third.