ST. LOUIS -- Noah Syndergaard's inability to pitch past the fifth inning for a second consecutive start bedeviled the Mets on Sunday, even as the offense responded with four solo home runs against the Cardinals.

Syndergaard hit one of those homers, but also allowed six runs (four earned) on eight hits and two walks in a 6-4 loss at Busch Stadium.

Pete Alonso, Robinson Canó and Michael Conforto hit the Mets' other home runs.

Though Syndergaard walked only two batters, both of those hitters turned into St. Louis runs. Jose Martinez scored on a Kolten Wong single in the second to tie the game at 1, and Marcell Ozuna was driven in by a Yadier Molina groundout in the third.

“I think today, the leadoff walks hurt him,” Mets manager Mickey Callaway said. “The two innings they scored multiple runs were both leadoff walks in the inning.”

Syndergaard concurred, saying that walking the leadoff man is “definitely something you don’t want to do.” He was blunt in his assessment of his performance both today and so far this season, even as he seemed to search for answers.

“To be quite honest,” Syndergaard said, “the entire beginning of this season, my performances have been unacceptable. I have to be better. I will be better. There’s no excuses for it.”

Syndergaard said he was having trouble repeating his delivery Sunday, and pointed all the way back to a 2014 stretch in Triple-A when asked to name a time at which he had similar struggles. Both Callaway and Syndergaard emphatically insisted that the right-hander isn’t dealing with any physical problems.

With runners on second and third, and one out in the second inning, Cardinals starter Dakota Hudson hit a slow-roller to shortstop that Amed Rosario was unable to field cleanly. The error loaded the bases and, two batters later, Paul Goldschmidt delivered a two-run single just past Rosario’s diving reach to put the Cardinals on top.

“When you don’t get the pitcher out, they can put something together,” Callaway said. “You really just shouldn’t give up crooked numbers when the pitcher’s spot’s going through there, because it should be an out.”

The error was Rosario’s fourth in the last five games, but Callaway said he wasn’t concerned nor did he see a need to seek out a day off for his shortstop. That could also be difficult depending on the status of Cano, who left the game in the seventh inning after being hit on the right wrist with a pitch that umpires ultimately ruled a swinging strike. Callaway was subsequently ejected for arguing that the call should have been a hit by pitch.

Regardless of the status of the infield, the Mets' main focus remains on a starting rotation that has a 5.64 ERA after Sunday's setback.

“[Syndergaard] knows he needs to get better,” Callaway said. “We need to get better as a pitching staff. That’s just the bottom line. We gotta pitch better than we are. And we will. I understand that guys go through tough times as a staff, and that’s what we’re doing right now.”