Although Dunn made the team out of Spring Training, he has had his struggles with command. Twice this season he was optioned to Triple-A New Orleans. He was recalled on June 21.
"I'm very excited because we're going to need him," manager Ozzie Guillen said. "We can't rely on Choate every time we face lefties. If Dunn continues to throw the ball the way he did the last two outings, that's going to be a huge move for us. That's going to be like trading for somebody."
Dunn has a 95-mph fastball, and against the Cardinals on Monday, he was able to locate it at the knees and on the corners.
During his second stint at New Orleans, he worked on his offspeed pitches and his slider, and he's added a curveball.
"Before I got sent down, basically the only thing I could throw for a strike was a fastball," the left-hander said. "They were laying off my offspeed pitches. Every pitcher goes down and you work on pitching in the bottom of the zone, and throwing quality strikes and not belt-high strikes. That's kind of where I was before.
"I was trying to throw to velocity and not to a location. I was throwing instead of pitching. It's as simple as that: Clear the head and go back and pitch and not throw."
A turning point in him finding his groove came on May 28, in his first appearance at Triple-A. Dunn was stretched out and threw 3 1/3 innings. He struck out six that night.
"It was a day game in New Orleans," he said. "It was hot. I couldn't go out there and just grunt my way through an inning. I had to go three innings. I had to settle and I had to pitch. Velocity was there. After that, it kind of clicked. I was like, 'Wow, that was pitching and not throwing.' "
In terms of balancing the bullpen, having two quality lefties gives the club an advantage.
"That's going to be the biggest move, or biggest add, we're going to have," Guillen said. "To have Dunn help Choate against lefties. ... With that arm, you've got to throw strikes. That's it. If we get to the point where Dunn can help us against lefties, this ballclub is going to be a lot better. We're going to need him."