"I talked to him this morning, and he feels good," Luhnow said. "He feels like he could throw today, but he's going to follow the plan, which is one more day off from throwing. He'll start throwing again tomorrow, and we expect he'll be ready to make his next start, which is penciled in for June 12."
Appel returned to the rotation at Lancaster on Saturday after spending a month at extended spring training, and gave up 10 runs and 10 hits, including three home runs, in 1 1/3 innings. Luhnow speculated the thumb was a factor.
"If you ask Mark, he's not going to blame anything on the thumb, but certainly it's something he felt the day after," Luhnow said. "You would have to assume it may have had some impact. I'm not worried about the control. The balls were all in the strike zone, for the most part. He threw close to 80-percent strikes. But when you talk about command, which is picking a spot in the zone and throwing it there, I think that's what was probably not ideal for him.
"Anything you have in your hand is going to affect your ability to have command. I'm not concerned, medically. I think he'll be fine, and hopefully he'll go out there on the 12th and do what we think he's capable of doing."
Appel's struggled to adapt to the tandem starter schedule at Lancaster earlier in the season. The Astros began the season using eight starters at each level of the Minor Leagues, though they recently went to a six-man rotation at Triple-A. Under the tandem system, each starter throws every fourth day and is followed in that game by another starter.
In four starts for Lancaster last month, Appel allowed 10 runs (nine earned) on 17 hits in 13 innings. He walked four batters and struck out 13. Last season, he went 3-1 with a 3.79 ERA in 10 starts between short-season Tri-City and Class A Quad Cities.
Brian McTaggart is a reporter for MLB.com and writes an MLBlog, Tag's Lines. Follow @brianmctaggart on Twitter. Mike Vernon is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.