Heading into spring camp this season, Phelps expected to compete for a bullpen role at either Daytona or Double-A Tennessee, but the right-hander soon began feeling discomfort in his left knee.
An MRI revealed a slight tear in Phelps's patella ligament.
"The only thing I can think of, I threw one pitch and landed completely closed. My foot didn't slide with me," Phelps said.
Phelps first noticed some minor pain in the knee last season. This spring, he says he was roughly two weeks from the end of Minor League Spring Training before the pain became too unbearable to pitch.
"I had a little flare-up last year and it never really felt 100 percent," Phelps said. "But this spring, it got really bad. It swelled up pretty big and there was a lot of fluid in there. I'm going to come back here once my season ends to have minor surgery and probably stay here until November."
In the meantime, Phelps is on a throwing program in Mesa and hopes to join Daytona some time in the coming weeks.
"I didn't even pick up a baseball for about two weeks after I was diagnosed with this," Phelps said. "The doctors had me in a knee brace to the point where I couldn't even bend my leg.
"I threw my first bullpen (Wednesday) and everything felt good," Phelps said. "I have another bullpen on Saturday and then some Extended games."
The Cubs drafted Phelps from Central Missouri State in the 11th round of the 2005 draft. He was ranked the Cubs' 20th best prospect by Baseball America entering 2006.