"He's one of the guys I'd think I'd consider kind of a key to our club," Mattingly said.
The Marlins have steady performers like Wei-Yin Chen and Jose Fernandez at the top of their rotation, already set to pitch the first two games, respectively. Tom Koehler has been the club's most durable starter the past two seasons. Cosart has the skill set to join the group.
"You count on Jose is going to be Jose," Mattingly said. "Chen has got a pretty good track record. Tommy is starting to form a track record. Jarred has always been a guy with really good stuff. It just hasn't quite panned out. Some of it no fault of his own."
In 2015, Cosart dealt with some freakish injuries, spending time on the disabled list with vertigo and an inner-ear disorder. During one stretch, he spent time at Triple-A New Orleans. Cosart ended up going 2-5 with a 4.52 ERA in 69 2/3 Major League innings.
Feeling unstable on the mound much of the season, Cosart averaged 6.1 strikeouts per nine innings. Once his inner-ear issue was resolved, Cosart finished up the season on a positive, posting a 2.96 ERA over his final five starts.
"The bump in the road last year is what it is," Cosart said. "But I feel like that's over."
In Spring Training, Cosart has been working with pitching coach Juan Nieves and vice president of pitching development Jim Benedict. They've modified the right-hander's delivery, stressing the importance of keeping his front side closed. They've also positioned him to create more of a downward plane on his fastball.
"I know what I can do, but at the same time, working with Benny and Juan, I don't know what I can do," Cosart said. "I haven't fully tapped into what I'm capable of."
Cosart has completely bought into what Nieves and Benedict are preaching.
"There's nothing not to like from Jarred this spring," Mattingly said. "Honestly, his stuff has always been that way. Can this guy take that next step? That's to be seen. We'll find out."
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.