By Bruce Levine–

CHICAGO (CBS) — Another big game from Willson Contreras was overshadowed Sunday when a Cubs team that has lacked consistency for much of the season saw its bullpen implode in a 9-4 loss to the Nationals at Wrigley Field.

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Up 4-1 after six innings, the Cubs faltered late, giving up eight runs in the final three innings, including five in the eighth. Six runs were charged to relievers, causing a strong outing from left-hander Jon Lester to go for naught.

“We didn’t play a very clean game,” manager Joe Maddon said after his team made two costly physical errors and too many mental mistakes.

“We absolutely have to turn up our defense, whether it is making double plays or throwing to the right bases. We are better than that. We work on it, we talk about it. It just has not been as tidy as it normally is. The other part has been the bullpen. We have good names there. We just have to get them back to being strike throwers and getting ahead on counts. Once we do that, we will be able to pitch like we can.”

The Cubs were in position to win largely because Contreras hit his 20th and 21st homers, continuing a recent surge that has pulled him into a tie for the team lead with 70 RBIs and tie the Royals’ Salvador Perez for most home runs by a catcher this season.

Afterward, all he was thinking about was dropping two of three in the series against a Nationals team that the Cubs almost certainly will face in the National League Division Series — if they make it there. Chicago leads Milwaukee by a half-game in the NL Central.

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“This was more mentally tough,” Contreras said. “We knew we were facing a really nice team. From what I see, somebody got a little nervous. It didn’t happen. I am not happy about it.”

Contreras’ “nervous” reference was in regards to Cubs reliever Carl Edwards Jr., who took the loss in allowing four runs, all earned, in 2/3 innings in the Nationals’ five-run eighth. That included a grand slam by Matt Wieters after he’d walked and hit a batter.

The Cubs were trying to prove they were the equal of the best clubs in the National League. They recently lost two of three against the Diamondbacks and have only won three series all season against teams that are above .500.

“We have a lot of information on the Nationals,” said Lester, who allowed three runs in 6 2/3 innings. “It is not about that. It is about execution. They did a better job than we did execution-wise. We were right in the game, but it got away from us a bit in the eighth. We were right in these games. It was not like we were getting blown out.”

The Nationals took the season series 4-3 from the Cubs. Now the question is whether the teams will square off again.

“This stings a little bit,” Rizzo said. “Teams break your heart. You break their hearts. It would be a good five games obviously (if we met in the postseason). It would be good. We play them well. That is baseball.”

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Bruce Levine covers the Cubs and White Sox for 670 The Score and Follow him on Twitter @MLBBruceLevine.