"He threw a lot of breaking balls yesterday," Stewart said Wednesday. "Mixed in his changeup, which is what we asked him to do leaving [Spring Training]. The biggest difference for him is he's commanding the zone with his fastball, pitching very, very well with his fastball."
The organization has been even more impressed with Lopez's maturity off the mound. Last season, his first since being signed following his defection from Cuba, he had a 4.69 ERA for Mobile and made just nine starts.
At one point during the season, he left the team without permission, but this year, he has been a model citizen from the first day of Spring Training, and his performance on the field had him in contention for a rotation spot at one point.
"He had a good Spring Training," Stewart said. "Showed well in Spring Training so I've always considered him an option -- he and [Braden] Shipley and [Tyler] Wagner, [Archie] Bradley, those guys were always options for us coming out of Spring Training. He's definitely improving himself in the pecking order, that's for sure."
Steve Gilbert is a reporter for MLB.com. Follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.