When Craig Counsell spoke to David Peterson over the phone Thursday morning, the Cubs manager could sense it in his newest pitcher’s voice. This was a lot to take in. Peterson had only known one team throughout his MLB career — and he was the longest-tenured Met. Just Wednesday night, during the second game of a doubleheader between the teams, Peterson was in the home dugout.

But that changed shortly before midnight, after the Mets sent the struggling lefty to Chicago in exchange for infield prospect Cole Mathis. Instead of spending the rest of his final season before free agency trying to fix what has gone wrong in Queens, Peterson was expected to start this weekend in Milwaukee for another team, potentially even flying with it after the series concluded at Citi Field and Peterson got everything in his personal life situated.

“Nobody tells you when you’re gonna be traded,” Counsell said. “They call you into the office or they call you on the phone and say, ‘You’ve been traded.’ That’s the warning you get. That’s shocking news for anybody. So I think he’s feeling the effects of that, and that takes a little while.”

And for the Mets, who entered play with a five-game losing streak and in the basement of the National League East, it marked the first move in what could be a summer filled with change to the current roster. Peterson’s deal could be just the initial trade. This is the reality for a spiraling team. After being an integral piece of the Mets’ improbable run to the NLCS in 2024 and after making the All-Star Game in 2025, Peterson’s trajectory stalled, with struggles forcing him to become a bullpen piece and bulk-inning reliever this season as a result.

“It’s sad to see him go,” Bo Bichette told The Post. “He’s been a great teammate so far in my time here and obviously spent a lot of time here, so yeah, I think it definitely sends some shock waves through the clubhouse.”

Mendoza said Peterson’s reaction to his role didn’t play a factor in the deal. He praised the 30-year-old’s ability to adapt to whatever the Mets needed. Peterson, a first-round pick in 2017 who debuted three years later, had collected a 6.08 ERA through 16 appearances — and eight starts — this season, with the latest clunker unfolding Sunday against the Phillies.

On a Mets team with so many rotation questions, Peterson was among the most puzzling. He pitched to a 2.41 ERA across the final two months of the 2024 regular season. He collected a 2.83 ERA across his first 20 starts the next campaign, too, and cracked the first All-Star roster of his career. He was supposed to be the constant. A source of stability in a 2026 rotation filled with inconsistencies (such as Kodai Senga) and unproven young pitchers (such as Nolan McLean).

“And then kinda everything went the other way,” Mendoza said.

After a three-start stretch where he allowed 14 earned runs in 14 ¹/₃ innings, Peterson was demoted to the bullpen. He made four starts after the demotion while also logging bulk innings in other outings.

“It’s a tough one,” Mendoza said. “because obviously, you understand this is a business. But especially from my end, I had a really good relationship with [Peterson]. … But it got to a point where needed the flexibility on the roster. Talking to David [Stearns], we already have Kodai in the pen and just continue to have starters in the bullpen, it’s just not gonna be sustainable.”

So in the visiting dugout pregame, Counsell talked about how he felt the Cubs could get the most out of Peterson and a fresh start. About how Chicago’s defensive strength — the opposite of a team like the Mets, who committed six errors Wednesday night — could benefit a pitcher who tends to induce plenty of ground balls. About how the Cubs were able to fill a need with five weeks still remaining until the Aug. 3 trade deadline.

The Mets, though, were left with a void. They were left with a new longest-tenured player. They were left with another subtle reminder that this could happen over and over again before the deadline arrives, even if Bichette, when asked about whether there’s a pressure to win before then, downplayed that reality.

“I mean, we’re just focused on winning as many games as we can,” Bichette said. “We can’t really focus on all that kinda stuff. We just have to focus on what we can do today.”