But he was one of four players added to the roster on Friday as part of the first wave of September callups. Others will join the team by Tuesday in Seattle.
"It's something that I wanted for my family, friends and myself," Mendez said. "It's a dream for me."
Mendez, 21, has pitched 292 innings in the Minor Leagues over the past five seasons after signing with the Rangers as an amateur free agent out of Venezuela in 2011. He pitched 66 1/3 innings for Class A Hickory in 2015.
He has close to the same number of Minor League innings that Nick Martinez (327), Nick Tepesch (300) and Chi Chi Gonzalez (224) had before they made their Major League debuts. Martin Perez had 494 innings in the Minors before his Major League debut, and Andrew Faulkner had 465.
"We will be mindful of where his innings are at," manager Jeff Banister said. "Obviously, there is a number we don't want to go over."
Mendez made 21 starts in the Minor Leagues this season between Class A Advanced High Desert, Double-A Frisco and Triple-A Round Rock. He also made three relief appearances, including in his last two outings.
"He is going to pitch out of the bullpen," Banister said. "He is a left-hander we saw in Spring Training, swing-and-miss secondary stuff, solid fastball. He can pitch one inning and he can pitch three innings. I [doubt] you'll see a situation where we'll run him out there on back-to-back days. Obviously, we'll get him out there at some point and see what he can do."
Banister said that Mendez's future is still as a starter, although he has been able to adjust to a relief role.
"It was a surprise after 20 starts to be a reliever, but I'm adjusting to it," Mendez said.
Over three levels, Mendez went a combined 12-3 with a 2.15 ERA while averaging 5.8 hits, 3.3 walks and 9.2 strikeouts per nine innings. He saved his best work for last. In four starts and three relief appearances at Round Rock, he went 4-1 with a 0.57 ERA. He averaged 3.4 hits there, along with 4.6 walks and 6.3 strikeouts.
"He's good," said Round Rock catcher Brett Nicholas said. "Really good fastball and changeup combination. That's what he lives off of, but he has the slider and curveball to keep them off balance. He works really hard. He doesn't talk a whole lot, but he wants the ball and goes after hitters. He challenges everybody that steps up in the box.
"You're going to like him."