CHICAGO — On a Monday night of reunions, when the reflex at Wrigley Field was to stand and clap, Jon Lester was on the wrong end of one homer, then another, then a third that stained his final line — a line of five earned runs in 5⅓ innings — and shot the Washington Nationals toward a 7-3 loss.

Both Lester and Kyle Schwarber were World Series-winning staples for the Chicago Cubs. And both were cheered before and during this matchup — except when Schwarber, the Nationals’ left fielder, smacked a two-run homer in the fourth, or when Lester was greeted by Jason Heyward, Willson Contreras and Javier Báez, the former teammates who took him deep.

“Didn’t really settle down until maybe the second inning, third inning, as far as all that stuff,” Lester said of the repeated ovations. “It’s just different, man. You’re pitching against your buddies. You’re pitching against guys that you’ve grinded with for a lot of years.”

Lester allowed eight hits and struck out four on 76 pitches. The Nationals (16-21) were outslugged at his expense and registered little offense in the late innings. They did have a chance in the eighth, with two outs and the bases loaded, but Starlin Castro was retired on a diving play by second baseman Nico Hoerner. That’s what sealed the defeat.

Castro, like Lester and Schwarber, was visiting his old park. He played six seasons and made three all-star teams in Chicago. And Washington’s Cubs connections run even deeper than that. Mike Rizzo, the Nationals’ general manager, grew up a Cubs fan on the west side of Chicago. Manager Dave Martinez was drafted by the Cubs in 1983, debuted with them in 1986, then was a bench coach here for three seasons, winning that title with Lester and Schwarber in 2016.

Alex Avila, their backup catcher, made a brief stop here in 2017. Welington Castillo, their third catcher and on the taxi squad for this road trip, was with the Cubs from 2010 to 2015. Second baseman Josh Harrison was drafted by the Cubs. Pitching coach Jim Hickey once held the same role for Chicago. Bullpen coach Henry Blanco was once on the Cubs’ staff, too.

But this was Lester’s and Schwarber’s homecoming. The crowd didn’t let them forget it. Lester, 37, was on the other side because, after six years and $155 million, the Cubs looked elsewhere to fill out their staff. Schwarber landed with the Nationals after the Cubs non-tendered him in December, despite having a year of team control left. These are business decisions that can still feel personal. Lester acknowledged as much this weekend, speaking about how it would feel to be inside Wrigley again.

“I think it’s natural, right?” Lester said. “ … The fans and the people on the outside don’t understand, a lot of times, what we do, and what we invest in the places that we’re at. Chicago was my home for six years. We have a house there, my kids have grown up there, you know what I mean? You invest in that city — your heart, your mind, your soul.”

“It’s not a hate thing or anything like that,” added Schwarber, who admitted he was glad to homer against the club that let him go. “It’s more of just going out there and proving something to yourself.”

That person, the one shaped by more than a half-decade in Chicago, took the mound around 6:55 p.m. Central time. The Nationals’ infield hung back for a moment, allowing the fans to shower Lester with a long and loud ovation. Lester took off his hat, quickly waved and went back to warming up. He wanted to focus on pitching. But once he could, and the Cubs stepped in the box, trouble stirred.

The Cubs (20-20) started with back-to-back singles by Contreras and Kris Bryant. Anthony Rizzo, the player Lester was most looking forward to facing, followed with a sacrifice fly to deep left to score Contreras. Next came Heyward’s two-run, opposite-field homer in the second. Then, in the third, Contreras cracked a solo shot in the same direction.

Lester was tagged with six hits and two homers in his first 38 pitches. His night was finished once Báez lifted a solo homer with one out in the sixth. Part of the damage was a credit to the Cubs’ approach — such as when Báez reached for a low-and-outside cutter that was a bit off the plate, or when Heyward drove a low-and-away fastball out to left. But Lester also permitted too much contact, missed some spots and created too big of a deficit for the offense.

“A little surprised,” Lester said of the Cubs attacking him early in counts. “But I kind of figured some of them would be a little bit uber-aggressive, just knowing me and knowing I’m around the zone.”

The bats did try to offer support. Trea Turner cranked his 10th homer, just nine shy of his career high. An inning later, in the top of the fourth, Josh Bell poked his second single and Schwarber hit that two-run blast to left-center off Cubs starter Adbert Alzolay. It was Schwarber’s 60th homer in this building. It was the first time, though, that the ball was chucked right back to the outfield grass.

Schwarber and Lester were only welcome to a certain point Monday. Heyward, Contreras and Báez, and a loyal fan in the left-center field bleachers, made sure that was clear.