Appel had already zoomed through short-season ball and said he didn't see much of a difference in his first shot at full-season competition.
"I think it's pretty similar in regards to talent," Appel told MiLB.com. "In short-season, most are college guys who signed this year, compared to younger guys here in [Class A]. But they're very talented. In regards to differences, it's still baseball. You still have to throw strikes -- that's my focus."
Appel's command had been spotless before his Midwest bow. On Sunday, however, he walked a batter for the first time in three Minor League starts.
"I left some of my off-speed stuff up more than I wanted to, and I missed with some pitches I usually like to see strikeouts on," Appel told MiLB.com. "But considering it was my third start in about seven weeks, I'm not overly concerned about anything."
Appel, a product of Stanford University, will celebrate his 22nd birthday on Monday. With his recent promotion to Quad Cities, he is a teammate of last year's No. 1 overall pick, Carlos Correa, marking the first time consecutive top selections have been on the same Minor League team.
"I think it's cool," Appel told MiLB.com. "Hopefully, it's something for the fans to get excited about. Houston fans can be excited about the future of the Astros. There's talent at the big league level and there's a lot of guys coming up in the next couple years."
Doug Miller is a reporter for MLB.com. Follow him on Twitter @DougMillerMLB and read his MLBlog, Youneverknow. This story was not subject to the approval of Major League Baseball or its clubs.