It will be the first major-league start for Perez, who pitched 2/3 of an inning in relief Wednesday night. Texas manager Ron Washington said the Rangers decided to go with Perez over Justin Grimm because he's left-handed.
Oakland is hitting just .222 against left-handed pitching.
Perez is excited about his opportunity.
"I just want to do my job," said Perez, who allowed four runs (one earned) against Detroit. "I just want to do the same I did in spring training, just throw the ball over the plate and see what happens. Attack the zone early and throw my breaking pitch for strikes. Throw quality pitches and help the team, that's what I want."
While Perez is the Saturday starter, there are no guarantees after that. Washington said the Rangers would use the off day to give all the starters an extra day of rest. Roy Oswalt will start Tuesday's series opener in Chicago and the Sunday before the All-Star break. The July 6 starter is still to be decided.
Derek Holland will start Monday for Round Rock and Washington said the plan is still for him to make three rehab starts, although that could change.
The Rangers hit in the batting cages for a third-consecutive Friday to try and avoid the heat.
Washington said he can tell a difference when his team hits inside vs. taking BP on the field on triple-digit heat.
"You can tell when we hit on the field, you really can," Washington said. "We're just sluggish for a few innings but we've got to do it sometimes. We just can't keep hanging out in here. We've got go get out there."
Washington said it's important for the club to take BP outside because it gives the infielders work and the players can run the bases. He also wants his club to get used to the Texas heat, which isn't going away any time soon.
The Rangers will take BP outside Saturday but not Sunday.