Laboring through a disappointing year, Cubs starter Edwin Jackson is already looking ahead to next season.

Jackson seemed to have his slider working early before things unraveled in another erratic performance that led to the Cubs' 6-2 loss to the Miami Marlins on Tuesday night.

"It's definitely been a disappointing season," Jackson said. "But every lesson is a lesson learned. You either accept it and be complacent with it or you find ways to keep working harder and get ready to bounce back. I have three more years here, hopefully. When you have seasons like this, it definitely makes you look forward to coming back and bouncing back for the next three years."

Jackson (7-15) held the Marlins scoreless through four innings, but gave up three runs and five of the seven hits he allowed in the fifth inning, his final frame of the evening.

Opposing pitcher Tom Koehler led off the inning with a single and scored on Christian Yelich's double. Giancarlo Stanton followed with an RBI single and Jackson couldn't get help from shortstop Starlin Castro, who committed his 17th error of the season after mishandling Justin Ruggiano's routine grounder. Yelich scored on the play to give the Marlins a 3-0 lead.

Cubs manager Dale Sveum said a pitch that was working for Jackson early suddenly disappeared in the fateful fifth.

"Had probably one of his best sliders early in the game and then that kind of got away from him," Sveum said. "Throwing it short and non-competitive sliders there. Not a lot of fastball command, up in the zone. Obviously a lot of foul balls, efficiency got away from him and then obviously the game (got away) there in the fifth."

The loss wasn't all on Jackson, as a Cubs offense that's failed to support the pitching staff all season was once again unable to add on after getting back in the game.

Brian Bogusevic got a run back for the Cubs in the fifth with his third homer of the season. Koehler loaded the bases with one out and was pulled for Dan Jennings, who allowed a run-scoring sacrifice fly to Anthony Rizzo to cut the Marlins' lead to one.

"It's disappointing when your team puts up three runs like that," Koehler said. "You want to get them back in the dugout as soon as possible. You give up the leadoff home run, things like that are going to happen, but I you got to be able to shut them down. It is one of those innings that can't happen. I just got to learn from it."

Koehler pitched 4 1-3 innings, allowing two runs and four hits, but was picked up by the Miami bullpen. After Koehler exited with the bases loaded in the fifth, the pen retired the next 11 batters and held the Cubs hitless the rest of the way, allowing only a lead-off walk in the ninth. Ryan Webb (2-5) pitched two perfect innings for the win.

Miami broke it open with a three-run eighth. Donovan Solano hit a two-run double - one of three hits on the night - off reliever Blaker Parker and Yelich followed with his third hit of the evening, an RBI single.

The four-run deficit proved too much for the Cubs bats, but Jackson refused to lay his poor performance on the offense's struggles.

"As a starter, you really can't worry about runs," Jackson said. "If you're a starter and you're out there worrying about run support, then you're probably not going to get too far throughout the game anyways. It's one of those things you can't control."