But when things started to go awry shortly after the All-Star break, he stood in front of a mirror and studied his motion to find out exactly what the problem was.
On Monday, the flaws he fixed in his delivery led to one of his best outings as a pro. Kleven scattered two hits over a career-high eight innings in the Class A Lakewood BlueClaws' 8-0 win over the Delmarva Shorebirds.
The native of Kamloops, British Columbia, struck out six batters, did not issue a walk and faced the minimum before turning the shutout bid over to the bullpen in the ninth.
"I had some struggles and I was trying to work my way back by figuring out what had been working for me to that point," Kleven said. "My mechanics had started to change or alter. I started doing some mirror work, going through my mechanics to see what didn't look normal.
"I noticed my arm slot has dropped a little bit -- that wasn't a big deal, I just got it back higher -- but I was leaning forward too much and not getting back over the rubber. That meant either the ball would be elevated because my arm would drag or it would be right down in the dirt."
With the mechanical tweaks behind him, he had little difficulty against Delmarva.
Kleven allowed two batters to reach base -- Michael Ohlman stroked a clean single to left field to lead off the second inning and Roderick Bernadina singled to right field to begin the seventh. Both hits were erased on inning-ending double plays.
With the victory, Kleven improved to 6-7 and lowered his ERA to 4.40.
He was at 71 pitches through eight frames, and he would have came back out for a shot at his first nine-inning complete-game shutout had the BlueClaws not scored four more runs in the top of the inning.
The 21-year-old's previous career high was seven, achieved four times, most recently in the BlueClaws' 8-3 win over the Hagerstown Suns on July 3.
"If the game was still 4-0, I would have gone back out there, but we got some more runs and they wanted to get the new guy [Chris Nichols] in there for some work," he said.
Kleven was 4-5 with a 3.80 ERA in 13 starts in the first half of the season. But since the All-Star break, he had gone 1-2 with a 6.39 mark in seven games entering Monday's contest. Even though his strikeout-to-walk ratio has been better in the second half, batters are hitting .302 against him, up from .253.
"I've had some good games and I've had some bad games," said Kleven, selected by the Phillies in the 33rd round of the 2009 Draft. "It's still a learning process for me, but I'm getting better every day. I'm not having an amazing season, but I'm working on the little things and learning more about myself and the game.
"Sometimes I get geared up too much and I'll try to overthrow or throw too hard or harder than I'm capable of. Or there will be something I can't control, like a bad call, and that will affect me."
Nichols worked a perfect ninth to complete the shutout.
Delmarva starter Eduardo Rodriguez (4-6) allowed two runs on one hit and two walks while striking out four batters over five innings.