Phillies manager Joe Girardi emerged from the dugout to defend his center fielder.

Roman Quinn got ejected after he argued a called third strike in the top of the third inning in Monday night’s 5-1 loss to the Nationals at Nationals Park. Girardi approached home-plate umpire Junior Valentine to discuss the borderline pitch at the bottom of the zone and express his frustration that he ejected Quinn while the outfielder was walking back to the dugout.

“Clean it up,” Girardi said. “This is a big game!”

It was a big game. It was an even bigger loss, because it dropped the Phillies to .500 and into eighth place in the eight-team National League postseason race with only six games to play.

• Box score

“We’ve got to win,” Bryce Harper said. “These are games that we’ve got to win, especially today. … There are teams right on us. They’re creeping on us fast. We have six games to get in there and get going.”

The Phillies’ urgency is what made Monday’s performance so disappointing. They managed only six hits against the Nationals. They committed three errors. Their only run scored when Mickey Moniak came home on a balk in the fifth inning.

They did not look like a team fighting for a postseason spot.

“We played poorly, really poorly,” Girardi said. “That’s what bothers me more than anything. We need to play better moving forward if we want to play in October.”

It was the second consecutive Zack Wheeler start that turned into a loss for the Phillies. It was Wheeler’s second start since he rejoined the rotation after nearly losing the nail on his right middle finger earlier this month. He allowed five hits, three runs (two earned runs) and three walks while striking out seven in 5 2/3 innings.

“I think it hurts more because our offense didn’t show up,” Harper said. “I think that’s the biggest thing. [Wheeler] shows up for us every single start. He’s out there competing each inning he throws. When you’re not able to put runs on the board and you lose ballgames, especially when a guy only gives up two or three runs … we’ve got to be better offensively. I think you guys saw that tonight.

“We should have been able to get all over Aníbal [Sánchez] today. We weren’t able to do that.”

The Phillies lined up their rotation so that Wheeler could pitch Monday and Aaron Nola could pitch Tuesday in Game 1 of a seven-inning doubleheader. It has Wheeler on course to pitch Saturday against the Rays in St. Petersburg -- and Nola on course to pitch the regular-season finale on Sunday.

The Phillies don’t want that, though.

“If we can clinch before the final game of the year, I think we can hold Aaron back and have him start Game 1 [of the postseason],” general manager Matt Klentak said before the game. “That’s the goal, but if we need him to pitch in the final game of the year, then he’ll pitch in the final game of the year.”

Losing Monday increases the likelihood that the Phillies will need to pitch Nola and Wheeler this weekend just to make the postseason. Since winning three games over the weekend against the Blue Jays, Philadelphia has lost two in a row. The Phils remain without Rhys Hoskins and J.T. Realmuto, although Realmuto could catch one of Tuesday’s games. They are without Jake Arrieta and Spencer Howard, which is why they are shorthanded in the rotation.

“We’re a team that has to go out there and play games, no matter what,” Harper said. “We have how many games left to win? We have to win and get into the first round and get going. There’s no time to complain. There’s no time to say, 'Oh, I don’t feel good,' or 'Poor me.' I don’t like that mentality anyway. We have to get out there and get going and play our game.”

The Phillies could still make the postseason if they finish 3-3, but going 4-2 (or better) down the stretch would certainly put them in a stronger spot.

“Every game counts,” Wheeler said. “I wish tonight was a different outcome, but we’ve got Aaron out there tomorrow. Hopefully we can start reeling off some games here.”