CHICAGO — After allowing a second-pitch home run and a single to the next batter, Cade Horton went into the cruise-control mode that Cubs fans had become accustomed to in the second half this season.

The Cubs rookie right-hander retired nine in a row in Tuesday’s series opener against the New York Mets as his teammates quickly staked him to a lead following his first-inning hiccup.

But Horton’s day was done after that frame, as he exited after just three innings and 29 pitches with what the team later announced was precautionary because of back tightness.

The Cubs’ bullpen proceeded to allow six unanswered runs, five of which came unearned in the top of the fifth inning following Dansby Swanson’s fielding error.

Initially, Horton’s departure seemed a concerning blow to the Cubs’ pitching staff, as the 2022 first-round draft pick is a potential candidate to start Game 1 of next week’s NL Wild Card Series. The way the Cubs had lined up their rotation certainly suggested Horton was a prime candidate for that start.

However, manager Craig Counsell revealed that Horton was dealing with an illness last week, which was a likely cause of the discomfort he felt in his mid-back near his ribs.

“Cade was sick this week — we’ve had a couple guys sick — and he was coughing a lot,” Counsell said following the Cubs’ 9-7 loss. “He noticed the symptoms on Sunday, Monday-ish — no effect throwing — and it just started to tighten up a little bit, so we pulled the plug on it.

“He just had symptoms on deep breaths. That’s what we feel like it probably is. We want to be extra careful.”

With five regular-season games to play after Tuesday, Horton’s next outing wouldn’t be until the playoffs. Matthew Boyd is scheduled to start Wednesday night’s contest, and Shota Imanaga is set to open Thursday’s series finale, potentially giving a glimpse into what the Cubs’ rotation in the best-of-three series might be.

That turns into a bit of a question mark if Horton’s back tightness lingers. But Horton, who was adamant about staying in the game Tuesday before the Cubs pulled him, is confident that he’ll be in good shape before he takes the mound again.

“(We) just wanted to get on top of it early, while we have time, rather than keep going and potentially harm the next start,” Horton said. “That’s why we made the move we did tonight.”

Horton had been on an absolute tear in the second half, with a 1.03 ERA and a 0.78 WHIP in 12 starts since the All-Star break. He not only has given the Cubs a chance to win each time he’s pitched, but he’s also dominated opposing lineups.

“He’s probably our most valuable player in the second half of the season, and he’s really carried us through a stretch where we haven’t always scored a ton of runs,” Nico Hoerner said of Horton after the Cubs’ loss Tuesday. “He’s put up quality start after quality start. More than that — dominance. He’s been amazing.”