Tretter has passed every test McCarthy and his coaching staff has put in front of the second-year pro. From the offseason work to the full-pads practices of training camp and through the first preseason game, there has been nothing to suggest that Tretter won't line up in front of quarterback Aaron Rodgers when the regular season opens on Sept. 4 at the Seattle Seahawks.
"Every day he's gotten better at something and he continues to grow that way," Packers offensive line coach James Campen said. "And a lot of that is because he knows what to do. He doesn't have to sit there and think and have that hold him back. He knows his assignments so well and knows what everyone else is doing, so that just accelerates his growth."
The Packers could not have asked for a tougher assignment for Tretter's first start last Saturday at Tennessee. The monsoon-like conditions made the ball tough to handle, but Tretter and quarterback Matt Flynn did not have a single problem with an exchange in their 16 snaps together.
It won't get any easier on Saturday at St. Louis -- although the weather won't be a factor in the Edward Jones Dome -- where Tretter will have to deal with the defensive tackle combination of Michael Brockers and Aaron Donald, a pair of first-round picks.
"I think JC's off to a great start," McCarthy said. "I think the Tennessee game was definitely impressive. I want to see him stack success anytime you play well. We're getting ready to play against an extremely talented, very good defensive front, so this will be great work for us."
Against the Titans, Tretter handled both his pass-protection and run-blocking duties with only one correction -- a technique mistake -- needed in the postgame film review.
"Talking with the other offensive linemen, they feel comfortable with him," said Flynn, who started in place of Rodgers. "It was nice that he played really well this past weekend. That was a good sign, and I think he's going to grow more and more and faster because of the two guards [T.J. Lang and Josh Sitton] around him."
But one game does not make an NFL center. Before last Saturday, Tretter had never played in an NFL game and had never snapped a ball at any level. He spent his entire college career as a tackle at Cornell and then did not play at all last season as a rookie after breaking his ankle in his first OTA practice.
"I still think I haven’t made the transition fully yet," Tretter said. "It's still a work in progress. It's kind of a mentality, it's kind of a mindset that we came into it understanding that there was going to be bumps. There are going to be days where it didn't look good but as long as we continued to correct our mistakes and build off our successes, I'd continue to become a better player and that's kind of how we went into it and that's how we kind of stayed throughout."