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.