CHICAGO -- When Cole Hamels walked off the field at Great American Ball Park in his final June outing, it provided one of the defining moments of the Cubs' season. Chicago was already struggling to surge forward in the National League Central race, and now one of the rotation's rocks had cracked.

Hamels was not the same after that left oblique strain on June 28, and that setback contributed to Chicago's issues over the rest of what finished as a third-place campaign. Before the season was over, Hamels wanted to do what he could to offer a different defining moment as he headed into free agency this offseason.

The Cubs declined to offer Hamels a one-year, $17.8 million qualifying offer on Monday, so he will hit the open market without a compensatory Draft pick coming for the Cubs.

"Trust me, I know I'm healthy. I feel amazing," Hamels said in late September. "I don't want to put that in the back of teams' heads of how I finished. I think I'm capable of what I was able to do in the first half. That's who I am. And I can still get those good results for, hopefully, it would be this team, if they consider that. But also, for other teams to know that I'm not the type of player that's on the regression."

Hamels, who will turn 36 in December, took the mound on Sept. 28 in St. Louis after missing a start due to left shoulder fatigue. The lefty then struck out eight and walked none in four shutout innings. It was not enough to erase his struggles over the final two months, but the performance did indeed give the Cubs and other clubs something else to consider as Hamels hits the open market.

Overall in 2019, Hamels went 7-7 with a 3.81 ERA and 143 strikeouts against 56 walks in 141 2/3 innings (27 starts). The lefty entered his ill-fated outing in Cincinnati with a 6-2 record and 2.92 ERA in his first 16 starts of the season. While both the rotation and bullpen dealt with issues, Hamels had been a reliable cog for the Cubs to that point.

"[The rotation] had a chance to be a real defining strength of the club, and it turned out not to be," Cubs president of baseball operations Theo Epstein said late last season. "Cole walking off the mound with that oblique strain ended up being a really pivotal moment in the season in some ways, because he performed a little differently once he came back."

Hamels believes he can perform much better in 2020.

"I enjoyed my team here and I'd like to continue it," Hamels said. "I do understand what they have to go through, construct, identify. But I would love to be a Cub. And I know, if it's not the case, at least I left it out here."

What went right?

Over the first three months of the season, Hamels was a steady presence in the Cubs' rotation. The lefty really hit his stride in June, when he fashioned a 1.22 ERA in six starts. In a four-start stretch from June 2-18, Hamels posted a 0.31 ERA with 31 strikeouts to six walks in 29 innings.

On June 18, Hamels became the 10th lefty in Major League history to reach 2,500 career strikeouts. He joined Randy Johnson (4,875), Steve Carlton (4,136), CC Sabathia (3,093), Mickey Lolich (2,832), Frank Tanana (2,773), Chuck Finley (2,610), Tom Glavine (2,607), Warren Spahn (2,583) and Jerry Koosman (2,556) on that list.

"It's a special moment," Hamels said after achieving that milestone. "It blows me away. I'm fortunate to be in this position. I obviously want to keep continuing and doing it as long as I possibly can."

What went wrong?

Hamels' remarkable June came to an unfortunate end with his oblique injury in Cincinnati. That cost the lefty roughly a month on the shelf, and he was not right after coming off the injured list. Upon his return on Aug. 3, Hamels posted a 6.39 ERA with a .949 opponents' OPS over his next nine appearances. He lasted fewer than five innings five times during that stretch.

"I wish I would've been able to do it a little bit better. I wish I was healthier," Hamels said. "I do feel like I let them down, let my teammates down, just because I know if I was at my best, I think the situation would be a lot different. So I do take that to heart. It's something that I'll be thinking about this offseason."

Best moment

Hamels turned in a vintage performance on June 7, piling up 10 strikeouts, scattering three hits and walking one in eight shutout innings in a 3-1 win over the Cardinals. In his career, Hamels has 20 games with at least 10 strikeouts and no more than one walk, tying him with Hall of Famer Walter Johnson for 28th on the all-time list for such performances.

"Some days you have to say, 'Man, that's some pitching right there,'" Cardinals manager Mike Shildt said after the game. "That's as good as anyone has thrown against us all year. He was in control, command of everything, changeup, put it where he wanted to. He had all his ingredients working."

2020 outlook

Last offseason, the Cubs picked up Hamels' $20 million team option, feeling that a one-year deal at that cost for a veteran pitcher of his caliber was a good addition. At the time, Chicago was also unsure how Yu Darvish's season would go, given his injury woes in 2018. The Cubs opted against a one-year offer this time around, sending the veteran Hamels into free agency for the first time in his career.