"I was just frustrated, it had been a bad day for me," Kemp said after the Dodgers beat the Cardinals, 5-3, at Dodger Stadium. "I didn't really do much to help the team win. It is what it is. It's over with now."
Mattingly removed Kemp from the game in favor of Skip Schumaker in a double-switch that brought Kenley Jansen in to relieve Paco Rodriguez.
The manager said he decided to remove Kemp from the game and not another position player in order to have Schumaker bat in the pitcher's spot, which was due up second in the bottom of the seventh, and allow Jansen to return for the eighth inning.
Jansen struck out Cardinals slugger Matt Holliday to strand two runners in the seventh, then struck out three more during a scoreless eighth to preserve a 4-3 lead. Schumaker went 1-for-2 with a single.
"It was the right move," Mattingly said. "This has nothing to do with Matt struggling. It was a baseball move. I don't like doing it, but I was at the point with the 'pen where I knew I needed to get one-plus [innings] out of Kenley."
Mattingly said he did not interpret Kemp's frustration to be directed towards him.
"If that's the case, we'll talk about it," Mattingly said. "I know Matt's frustrated with things, but there was nothing personal there. I love Matt. It was a baseball move. Guys don't like coming out and I understand that."
Kemp was also booed by the crowd after striking out in the sixth with runners on first and second.
"That's how they feel," Kemp said. "Let them do what they want to do."
Austin Laymance is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.