The Marlins have yet to announce all five members of their rotation. They have tabbed Wei-Yin Chen to pitch Opening Day and Jose Fernandez for Game 2. Tom Koehler has earned a spot, perhaps at No. 3, but that could change.
Basically, two places have not been filled. However, it appears lefty Adam Conley and Cosart have secured the final two spots.
Cosart has enjoyed a strong Spring Training, not allowing a run in 9 2/3 innings with nine strikeouts and five walks.
With pitching coach Juan Nieves looking on Saturday, Cosart came out pounding the zone with fastballs, and he struck out two of the first three hitters he faced.
After that, Cosart went more with offspeed pitches. At one point, he even adjusted the grip on his changeup in mid-inning. So, he ended up issuing a few walks, and he hit two batters. Cosart also struck out five.
"The first inning, [Nieves] wanted me to go out there and get the fastball right, and pitch like I'd pitch in a Major League game," Cosart said. "After that, the results might have changed a little bit.
"We've been working all camp on this new delivery, so I wanted to implement throwing two, three, four breaking balls in a row, or three or four changeups in a row and not really mixing it up. It was just more quantity over quality. I felt good with it."
In Cosart, the Marlins see tremendous upside, and Saturday's performance reinforced what he's shown all Spring Training.
The delivery adjustment he's working on is basic, keeping his front side closed longer. Some of the tweaks are enabling him to get more downward movement on his fastball.
"There are two or three things going on," vice president of pitching development Jim Benedict said. "We're not trying to do them all at once, but incrementally to get him [throwing] downhill] and getting all of his pitches in the zone. Nothing outside of the fundamental type [of] things. But he's taking to them, so we're encouraged.
"He's been nothing but everything we hoped for, as far his ability and matching up his ability with his mentality. We're trying to get his physical gifts out."
• Saturday's Grapefruit League game against the Astros at Osceola County Stadium in Kissimeee, Fla., was canceled following a lengthy rain delay.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.