ATLANTA -- The Phillies do not have a No. 5 starter, so Sunday they hoped to mix and match their way to victory at SunTrust Park.

They told Vince Velasquez late Saturday night that he would start the series finale. They made the announcement Sunday morning. A few hours later he allowed three runs in the first inning, and Cole Irvin, Philadelphia's No. 16 prospect, allowed six in 3 2/3 innings in a 15-1 loss to Atlanta, featuring a scoreless eighth inning from Phillies position player Sean Rodríguez. The Braves won the series to take a 2-1/2 game lead in the National League East. The Phillies lost for the 10th time in 16 games. They open a four-game series Monday night against the Nationals in Washington.

“We have to be better every time through the rotation, there's no question about it,” Phillies manager Gabe Kapler said. “We're going to need to perform really well against teams like the Atlanta Braves. They are a deep lineup. We've obviously played them well overall in the year, but today's effort wasn't good enough.”

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Jake Arrieta, Zach Eflin, Nick Pivetta and Aaron Nola will pitch against the Nationals, who have won 14 of 21.

But who pitches Friday night, when the Phillies return home to face the Marlins at Citizens Bank Park? Velasquez? Irvin? Jerad Eickhoff? He held the No. 5 job last week, but he got bumped to the bullpen because he had allowed 16 home runs in 27 1/3 innings with an 8.89 ERA in his past six starts. He allowed two home runs and four runs in the seventh inning Sunday.

What about Triple-A right-hander and No. 6 prospect Enyel De Los Santos or No. 2 prospect Adonis Medina, currently in Double-A?

Do the Phillies try a true opener, like left-hander José Álvarez? If so, who follows him?

More important, can they survive like this in the long run?

“Whether we have a fifth starter or we don’t, we should all be capable of putting up zeros and [be] able to be the next guy up to answer the call,” Irvin said. “Yeah, disappointing, obviously. It’s an embarrassing way to end the series here. We had two good games early. It’s unfortunate. I felt like I let the team down today.”

The Phillies’ rotation entered the afternoon 13th in baseball with a 4.34 ERA, 18th in xFIP (4.55), 27th in FIP (4.91) and 27th in WAR (2.7), according to FanGraphs. They recently passed on free-agent left-hander Dallas Keuchel, who signed a one-year, $13 million contract with the Braves on June 7.

Keuchel could make his Braves debut Friday.

The Phillies passed on Keuchel because they said at the time their rotation had been solid and healthy. They liked their depth, too. But it seems more and more like the Phillies need to trade for a starting pitcher, if they expect to hang around in the NL East. There should be some available, including Madison Bumgarner, Marcus Stroman and Matthew Boyd.

“We haven't pitched our best recently,” Kapler said. “I think that we have a better level of play in us in totality, and I have trust in our starting pitchers -- Nick, Jake, Nola. Obviously Eflin has been outstanding. We have a group of guys who have a track record of success, and Nick has been sensational since he's back from the Minor Leagues. There's some confidence there.”

Mike Foltynewicz could be considered the Braves’ No. 5 starter with the way he has pitched this season, but he allowed one run in six innings. He faced a lineup without Jay Bruce and J.T. Realmuto. Bruce rested a tight left hamstring. Realmuto need a break after being hit between the legs by a foul ball Saturday.

The Phillies hope to have everybody healed and playing better when they return to SunTrust Park on July 2 and open another three-game series against the Braves. The Phillies play the Braves 13 more times this season.

“I think we'll rebound from this with ease,” Velasquez said. “I don't have any hesitations or doubts in myself or as a pitching staff, in general. I think it's just one of those games where these guys are hot, and we've got to tip our caps to them and keep moving forward. They had a solid month, and we're right on their tails.”