"I don't think you want to pitch on Tuesday," Kazmir said he told Gray in a text message.
Kazmir pitched alongside Gray in 2014, and the first half of 2015. Kazmir said he and Gray grew close during their time together in Oakland.
"We do have a close relationship," Kazmir said. "We were a tight-knit group. We were really close, doing everything on and off the field together."
Kazmir said as soon he was dealt from Oakland to Houston, his Athletics teammates started to poke fun at his eventual return to the mound at O.Co Coliseum.
"As soon as I got traded, [there] was trash talking," Kazmir said. "It was, 'I can't wait 'til you get back here.'"
But the 11-year veteran knew the kidding would reach new heights once it was set in stone Gray would pitch against him.
"With Sonny up there, he's already been texting," Kazmir said. "Once we figured out exactly when he was going to pitch, that's when the trash talking started."
Kazmir said it was strange to be in the visitors' clubhouse at the Coliseum when the two teams met in early August, but this time around he's just trying to "take care of business."
The 31-year-old did not have a chance to face his former team during that four-game series, but said he's excited to be able to face his former team Tuesday.
"They know me, I know them, so it's going to be fun," Kazmir said.
Athletics manager Bob Melvin, who said he was not surprised by the verbal battle between the two friends, took a more serious approach when discussing their relationship. He touched on how big of an influence Kazmir had on Gray during his time with the A's.
"I don't know that there's anyone in the game Sonny looks up to more than Scott Kazmir," Melvin said. "He's done a lot for him, not only on-field stuff with preparation and so forth, but preparation too. They've become very close, so it should be a good matchup tomorrow."
Oliver Macklin is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.