"I'd be lying if I said I wasn't nervous in the first inning," Cosart said. "You kind of forget what a big league stadium is like after a month and a half of being gone."
After the first batter, Cosart limited the Phillies to just three hits and struck out three. The right-hander put in a good showing starting in place of Wei-Yin Chen, who landed on the disabled list on Sunday with a left elbow sprain.
Cosart unveiled a different windup than the one he had in April. Now, he starts from a stretch position, which he said simplifies his delivery.
"It just felt comfortable," Cosart said. "I don't have to move when I go to the stretch. I'm already like that, so it's not a huge change for me. It's something I kind of ran with and something I feel like I can not tinker too much with but refine some things and hopefully perfect it."
After going 0-1 and posting a 7.98 ERA in his first three starts for the Marlins this year, Cosart knew he had to change something. He spent time on the Minor League DL with an oblique injury, which gave him time to watch guys with similar windups.
Cosart studied film of pitchers like David Price, Jake Arrieta and even his opponent Monday night, Jeremy Hellickson.
"I tried everything," Cosart said. "I knew I was gonna be down there for a little while. I had a lot of different looks in my windup and stretch. I watched multiple video sessions."
The work paid off, and it showed against the club that selected him in the 38th round of the 2008 Draft. The Marlins traded for Cosart in 2014, a year that he said his mechanics were the best they've ever been.
"It's pretty similar to where it was in 2014," Cosart said of his windup. "When I came over here in the second half, that's the most success I've had in my career. I was trying to get back and replicate what I did then."
Patrick Pinak is a reporter for MLB.com based in Miami. This story was not subject to the approval of Major League Baseball or its clubs.