Gabe Kapler returned Monday to his office in the bowels of Citizens Bank Park and flicked on the two flat-screen televisions that face his desk.

The two screens, as they always do, flashed the Phillies schedule. And they reminded the manager, riding high from a successful road trip, how challenging the task ahead would be.

“I would say it is a very challenging stretch,” Kapler said before opening a four-game series with Milwaukee, which ended Thursday with the thud of an 11-3 loss. “There is excellent competition out there. This stretch is going to be a grind. There’s no sugarcoating.”

The Phillies lost three of four to the Brewers for their first series loss in three weeks. It was the beginning of a stretch of 20 games in 21 days against the Brewers, Rockies, Cubs, Cardinals, and Dodgers. And the Phillies began it with a rather dull four days.

Zach Eflin, who had two complete games in his last three starts, was ordinary. The lineup, tweaked to move Bryce Harper to the No. 2 spot, had just four hits. Seranthony Dominguez, who has been more erratic recently than electric, allowed a three-run homer in the seventh to make a third-straight loss feel certain before relievers Edgar Garcia and Austin Davis allowed four runs to seal it.

“If you don’t do your job, you’re going to get your butt kicked. That’s plain and simple,” said Andrew McCutchen. “You play good ball and you win. We can all agree here that we didn’t play our best baseball and in return, we got our butt kicked.”

In their three straight losses, the Phillies played poor defense, struggled on the mound, and failed to produce much at the plate. They scored three or fewer runs and combined for just five extra base hits. The lineup, constructed to thump opponents enough to overcome the deficiencies on the mound, has gone cold just in time for a grueling stretch. Their home run rate in May — 0.64 per game — is less than half of the rate they homered last month.

“When we were really clicking, or clicking better, as an offense, you’d get production from each part of the lineup — top, middle and end," Kapler said. “Right now, we’re not getting that. Again, it’s not a switch that you can flip to turn it on. If we could, we would do that. But there’s a lot of work to be done and we will take action steps. Happy to fill you in on those when we figure out what they are.”

Christian Yelich homered twice on Thursday and Ryan Braun reached base four times. Braun’s .417 batting average at Citizens Bank Park is the highest by a player with at least 140 at-bats. Yasmani Grandal had a three-run homer and Mike Moustakas had three hits. The Brewers possessed a deep lineup that offered little breaks, resembling the order the Phillies have when their offense produces.

Eflin allowed four runs on seven hits in five innings while striking out seven and walking two. His damage could have been worse, but he held his ground with runners on base in the third and fifth innings. Eflin was not the pitcher who has flashed such promise this season, but he still kept the Phillies afloat.

“Just have to be better,” Eflin said. “I’ve got to show up better prepared to execute the game plan. Didn’t really do a good job of that today. These next four days leading up to my next start, I’m going to work hard, study and get back on the right track.”

The Phillies are still in first place atop the National League East. But it will be this stretch that could determine how long they can stay there.

The Rockies, who took three of four from the Phillies earlier this month, visit South Philly this weekend and are 17-10 since starting the season at 3-12. The Phillies then travel to play the Cubs, who are 15-6 at Wrigley Field and the owners of the second-best record in the National League. They then visit Milwaukee for a rematch with the Brewers, return home to play the Cardinals, and fly west to play the National League-leading Dodgers.

The Phillies don’t have a day off until Memorial Day and they don’t play a team with a losing record until June 7. After a strong start to the season, the challenge is here.

“I don’t think that if you look at our club on paper that we’re not as good as the team we just faced,” Kapler said. “I do think they’re a very good baseball team, and we have to play very good baseball to beat them. We have to play good defense because we can’t give away outs. We have to execute. We have to execute the fundamentals, and we weren’t able to do that.”