Pitcher Mark Appel, the Houston Astros' top prospect, underwent an appendectomy on Thursday but is expected to recover in time for spring training, the Astros announced Friday.

Houston's pitchers and catchers report to camp on Feb. 15.

"Mark will be fine," Astros general manager Jeff Luhnow said in a statement. "He will report to camp on time. Once he arrives in spring training, we'll see where he is at health-wise and take it from there. We anticipate that he will either be ready to work out at the start of camp or very close to ready."

The 22-year-old right-hander was the No. 1 overall pick out of Stanford in the June 2013 amateur draft.

After signing a contract, Appel made two starts for low-Class A Quad Cities and eight starts for Class A Tri-City. He posted a combined 3-1 record with a 3.79 ERA in 38 innings.

The Astros saw enough of Appel to invite him to spring training. He is not expected to make the major-league club right away but could make his debut sometime during the 2014 season.

Astros left-hander Raul Valdes, who was claimed off waivers in October from the Philadelphia Phillies, underwent arthroscopic surgery on his left knee this week but is expected to be in camp despite an estimated recovery of four to six weeks.