Valverde and the Tigers have not been given an official warning, but MLB's handling of the incident indicates a de facto warning.
An internet video showed Valverde putting his glove, with the ball in it, to his mouth before delivering a 1-2 pitch to Devin Mesoraco during the ninth inning of Sunday's 7-6 victory over the Cincinnati Reds. Mesoraco swung and missed at the 94-mile per hour fastball.
"It's nothing. I never did anything wrong," Valverde said Tuesday. "I put my glove on my mouth, yeah. Why? Because I sweat too much. That's it."
Valverde said he saw the video and anticipated the media's reaction, laughing as a group of reporters approached him in the clubhouse.
"Everybody can say whatever they want to, but I wasn't doing anything wrong because I've never spit on the baseball," he said. "Everybody said what I was doing, but when you play in a stadium like that, Cincinnati is so hot. I sweat too much."
Valverde is 3-1 with 12 saves in 15 chances. He has a 3.91 ERA.
Information from the Associated Press was used in this report