But once he started to throw, his nerves tumbled as easily as the San Antonio hitters and MLB.com's No. 36 prospect allowed two hits over six scoreless frames, but did not factor into the decision as Corpus Christi lost to San Antonio, 4-0.
Cosart struck out four without walking a batter, facing one batter over the minimum in his six innings for the Texas League affiliate of the Astros.
"A little pressure, I was a little nervous at the beginning," he said. "But you go out there and throw your first warmup pitches and then it's another game. We've got a great group of guys. I just wanted to go out there and pitch like I know I'm capable of. It worked out pretty well."
The 21-year-old right-hander allowed only two baserunners in the game, retiring six and seven in a row at two separate points.
Anthony Contreras recorded the Missions' first hit when he singled to lead off the third, but moments later he was caught by Cosart attempting to steal second. The only other batter to reach base against the Astros' top prospect was Jason Hagerty, who grounded a two-out single through the left side in the fifth.
"I was mixing up all three pitches, but from the beginning, I was using my fastball," Cosart said. "My fastball command was good -- I was able to move it in and out, and they were swinging. It's always good when you're able to show one or two pitches early from a pitcher's standpoint, so hitters don't see it all."
The 38th-round selection by the Phillies in 2008 had gone 9-8 with a 3.92 ERA in 108 innings for Class A Advanced Clearwater this year. He was sent to Houston on Friday along with Jonathan Singleton and Josh Zeid in a deal for Major League outfielder Hunter Pence.
Cosart said he knew something was up in the seventh inning of the Threshers' game Friday, when Singleton -- MLB.com's No. 37 prospect -- was pulled.
"I had heard from my agent about the possibility that Singleton was gonna go," Cosart said. "It actually took about an hour-and-a-half [for them to tell us]. They were still in the process of finishing it. I got a call from [assistant general manager] Chuck LaMar, then [GM] Ruben Amaro."
Born and raised in Texas, Cosart said he was pleased about going to the Astros after the initial jolt of being traded had worn off.
"It was actually a little bit of shock and disbelief at first," he said. "It's kind of like a storybook thing, playing for your hometown team. I was kind of in shock at first, but then that turned to excitement. You see they've got a lot of young guys there and a willingness to move young guys up."
Cosart said he enjoyed his time with the Phillies, citing the staff and the numerous friends he made in the organization. But he realizes he probably can make the Majors quicker with the Astros.
"As a player, the No. 1 goal is getting to the big leagues," Cosart said. "I didn't think I'd have a chance with the Phillies for at least a couple years. They spent $250 million on three pitchers. Coming here, with a lot of young guys, it's a good opportunity."
Facing off against Cosart on Tuesday was Robbie Erlin, who was packaged with fellow Rangers prospect Joe Weiland in exchange for Padres reliever Mike Adams on Sunday. The 20-year-old southpaw allowed five hits over six scoreless innings, striking out six in his debut for the Padres.
San Antonio's Jorge Reyes (6-3) earned the win by relieving Erlin with two scoreless innings.
Zeid (0-1), another player making his first appearance with a new team, took the loss for Corpus Christi after surrendering four runs on four hits in 1 2/3 innings.