In his first Triple-A outing, Rosa gave up five runs in three innings of a loss.
"It was a different level, and the first time I got in trouble by keeping the ball up," Rosa said. "Nothing's changed, the game is the same everywhere. You've got to keep the ball down."
That lesson reinforced, he got better in each subsequent game. In fact, in his last outing on Saturday, he had four perfect innings against Iowa before rain washed out the game. He went 2-1 with a 3.96 ERA at Omaha, with an impressive 20 strikeouts and just four walks in 25 innings.
His best pitch is his fastball, but he just smiled when asked if it had much sink or movement.
"I don't know. I haven't tried to hit it, so I've never seen my fastball move," he said with a chuckle. "But I try to get the ball down and see how it works."
Rosa also throws a slider and a changeup.
"I have a better changeup than last year. I've been working on that since Spring Training, especially against the lefty. That's my second pitch against lefties," he said.
Rosa, who replaced Brett Tomko on the roster, will be working out of the bullpen for the Royals after being strictly a starter in the Minors.
Manager Trey Hillman was impressed with the way the ball seemed to explode out of Rosa's hand during Spring Training. His low walk totals are a big plus, too.
"The command and the control are there, and we'll see if he can continue to do that at the Major League level," Hillman said.
"I'm sure he's going to be a little bit nervous -- that's why I want to monitor exactly how we use him, especially his first time out."