Petrick remained in Mesa, Ariz., on a rehab program when camp broke and full-season Cubs minor league rosters were finalized late last month.
He is listed on the disabled list at Class AAA Iowa and said earlier this week that he hopes to rejoin the team in the near future.
"I pitched in a game a couple of days ago and it (shoulder) felt great," said Petrick, who underwent shoulder surgery in 2005. "It's feeling better every time out."
A year ago, Petrick began the season at Class High-A Daytona before moving on to Class AA Tennessee and eventually Chicago for his major league debut on June 27.
Petrick would close the season at Iowa, logging over 60 innings from start to finish in 2007. He finished the season with a 3.80 ERA in 41 appearances.
Shortly after a two-week stint on the disabled list, Petrick was shut down in late August with arm fatigue upon showing a noticeable drop in velocity. In the off-season, the Morris High School product visited with an orthopedic surgeon, Dr. Craig Morgan in Wilmington, Del., to receive a shoulder exam.
This spring, Cubs Vice President of Player Personnel Oneri Fleita said the club was just being precautious with Petrick and that he was not injured.
"He was just a bit behind the others," said Fleita.
To help speed up his return, Petrick has tinkered with his mechanics. When healthy last season, his fastball was in the low 90s consistently and topped out at 94 mph.
"(My pitches) weren't coming out of my hand very well," Petrick detailed. "I was cutting the ball off a lot. I worked on some easier mechanics to throw harder and with easier motions. ... My velocity is about 89 to 90 right now. Hopefully it rises from there."
Petrick couldn't guarantee an exact date for his return, only that, "I'm about to get out of here."
He said: "I don't think they ever tell you (a date) for sure, for different seasons."