After all, he hasn't pitched in the Major Leagues since 2006 because of an oft-injured right shoulder, and he's admitted he's thought about giving up on the game he loves in the past.
But Prior, who a posted a 42-29 record and a 3.51 ERA in five seasons as a young pitching phenom with the Cubs from 2002-06, isn't ready to throw in the towel just yet.
Prior, 29, signed with the Orange County Flyers of the independent Golden League on Tuesday and threw a scoreless inning in relief that same day in his first competitive appearance in nearly four years.
He said he hopes to get noticed by big league scouts and make a comeback as a reliever this time around after setting the National League on fire as a starter in his first go-round.
"I know they're out here watching," Prior said. "I'm just here to get some innings and get some work while getting used to playing in games. I want to show that I can be durable and that I can go out there and withstand pitching on a few days' rest or in back-to-back games like a normal reliever does."
But Prior is anything but a normal reliever. He entered the Majors in 2002 as a much-heralded right-hander -- much like the Nationals' Stephen Strasburg -- after being drafted as the No. 2 overall pick behind Twins catcher Joe Mauer in the 2001 First-Year Player Draft.
Prior won the Golden Spikes Award as the top amateur player while at the University of Southern California and was seen as baseball's top can't-miss prospect.
And Prior certainly didn't miss, at least at first, as he posted a 3.32 ERA as a 21-year-old rookie before going 18-6 with a 2.43 ERA in his second year in the Majors, being named an All-Star and finishing third in the balloting for the NL Cy Young Award.
But he pitched just 118 2/3 innings in his third season, as the shoulder injuries began to mount before bouncing back with a 3.67 ERA in 166 2/3 innings in 2005. In 2006, however, Prior struggled with a 7.21 ERA over just 43 2/3 innings before undergoing surgery on his right shoulder.
It's been a battle for Prior ever since, as he signed two different one-year contracts with the Padres in 2008 and 2009 but never pitched in a game because of setbacks with his shoulder.
He's heard the reasons for why he's been so injury prone -- too many innings as a young pitcher, getting into a collision with Marcus Giles in 2003 and getting hit by a line drive on right his elbow off the bat of Brad Hawpe in 2005 are just a few -- but he can't pin the exact reason.
"I don't think anybody knows what it is," Prior said. "There are a million theories out there about why I've been hurt. Was it the collision, was it the innings, was it the line drive? All those things probably played a factor in their own little part or maybe it was just one of those things. It's a game that grinds on pitchers and those who make it unscathed are few and far between."
But Prior feels that those health problems are in the past, as he's been throwing just about every day since last September and even threw in front of scouts at USC on June 30.
He said he's no longer rehabbing his shoulder and that he's preparing to pitch just like any reliever would in the Major Leagues.
"Right now, I feel great," said the 6-foot-5 Prior, who said he weighs 220 pounds. "It's probably as good as I've felt since 2005. I'm just trying to put as much work in as I can to stay healthy. I'm trying to come back as a reliever and go that route just to get my feet in the door and see how my body responds."
Prior was all smiles talking about his first relief appearance with the Flyers on Tuesday when he pitched a 1-2-3 inning, needing just 11 pitches to get through the frame. The competitive juices were there and Prior remembered how much fun it is to retire opposing hitters.
"The main thing is that I still have a passion to play the game and I'm still 29, not 39," Prior said. "I know how to pitch and get guys out so it's just a matter of going out there and doing the things physically that I want to do mentally."
But Prior is still trying to get back to where he was physically as his fastball that used sit in the mid-90s is now topping out at 90 mph, while his hammer curveball is at 76 mph when it used to be in the high-70s to low-80s.
"I don't expect to come out here Day 1 and throw 95 mph," Prior said. "I don't know if I'll ever throw 95 again. But if I can get in the 90-92 range and sit there, then that's doable. The game is about location and changing speeds. Velocity helps you get away with mistakes but there are a lot of guys who throw 90 that are still getting guys out."
His manager, former big league pitcher Paul Abbott, said it's been a thrill working with Prior so far. He also said Prior's return had been in the works for a while because Prior's agent John Bogg's son, Steve, is the Flyers' center fielder.
"This is a good springboard for him to showcase himself," said Abbott, who went 43-37 with a 4.92 ERA over 11 seasons in the Majors. "He can show teams that he's got the same stuff he had before he got hurt. I really believe they want to see him throw and log some innings and see him not break down before they give him an opportunity."
So while Prior is excited about the opportunity to be back on the mound in the Independent League, he's hoping that next season he'll back in the Majors even if he comes back as a reliever. But he knows even that is a long shot, so for now he's just going to try to stay healthy and see what the future brings.
"I just want to show teams that I'm healthy," Prior said. "I know what they see on paper so I want to show them it's a different story when they see me in person. The ultimate goal is to get to the big leagues just any other kid in college or in the Minor Leagues. I still think I can get to that level."
Rhett Bollinger is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.