"Christian Yelich for NL MVP," Mayfield said.
Before the game, Mayfield showed up to FirstEnergy Stadium wearing Yelich's Milwaukee Brewers jersey. The two became friends while working out at the same facility last winter.
Yelich, who is hitting .320 with 31 home runs and 93 RBIs as the Brewers chase a playoff spot, told reporters Friday that Mayfield's postgame gesture was "a cool moment and much appreciated" and explained how the top pick in the NFL draft came to acquire his autographed jersey.
"He came in one day wearing it," Yelich said. "He asked me if I would sign it for him. Obviously, I did, and he ended up wearing it to the game. You don't really think it was going to go down how it went down [Thursday], so it was a really cool moment for him getting that win for the Browns and the city of Cleveland."
Yelich didn't wear a Mayfield jersey to the ballpark before Friday's 8-3 victory over the Pirates, but he said, "I have one hanging in my locker in Milwaukee. I should have packed it. Bad timing on my part."
Yelich said the Brewers were rooting for the Browns as they watched the Thursday night game in Pittsburgh.
"They played a great game yesterday, so it was fun to watch," Yelich said. "We had a lot of guys in the clubhouse rooting for the Browns yesterday. Obviously, it's a big victory for the Browns and the city of Cleveland. It was fun to watch."
The Associated Press contributed to this report.