2017 stats (with Miami): 55.2 IP, 1-5, 7.92 ERA (6.91 FIP), 1.73 WHIP, 7.1 K/9, -1.4 WAR (Baseball Reference)
Let's be honest, Tom Koehler was never more than, or expected to be more than, a mid- to late-rotation arm for the Marlins in his four full seasons with the organization before 2017. His best year came in 2014, when he went 10-10 with a 3.81 ERA over 32 starts, and even that production was slightly below league average (3.74 ERA that season).
However, Koehler consistently started games for Miami, posting three straight seasons of at least 175 innings and 30 starts between 2014 and 2016, and he provided value as the workhorse of the pitching staff. He was expected to fill that role once again in 2017, and while things started well for the right-hander, with a one run outing at Washington followed by a quality start against Atlanta, the veteran's season quickly turned sour.
Koehler allowed 26 earned runs over his next 27.1 innings, including eight runs in just three innings against the Astros on May 16th, to send his ERA on the season to a sky-high 7.04, which was the highest among qualified starters at that time. An ongoing issue could have been the explanation for the string of poor performances, as he was assigned to the disabled list for the first time in his MLB career two days later with a shoulder injury, and he would not return to a major league mound until July 1st (when he promptly allowed six hits and seven runs in an inning and two thirds).
Following another three starts in which he averaged just over five underwhelming innings, Koehler was sent down to Triple-A New Orleans, and that is the last time that Marlins fans would see him. On August 19th, he was traded to the Blue Jays along with cash for 22 year-old right-hander Osman Gutierrez, who posted a 7.85 ERA over 18 Single-A starts in 2017. Koehler pitched 17 innings for Toronto, primarily as a bullpen piece, before entering free agency after the season and signing with the Dodgers.
After years of being an average starter for Miami, Tom Koehler became inconsistent and undependable for the first time in his career, and that was enough for the Marlins to cut ties with him. Despite the swift divorce, Koehler sits joint fifth on the all-time starts list for the Marlins with 132 and seventh with 767.1 innings pitched, which is a testament to his durability over the years. While not always dominant, Koehler more often than not did what a back-end starter had to do in Miami, and the workhorse mentality may be hard to replace in the short-term.