MILWAUKEE — Maybe it was the grind of a 10-game road trip. Perhaps it was the jarring news that manager Terry Collins had left the ballpark because of illness. Whatever the reason, the Mets turned in one of their sloppiest performances of the year Sunday.

In a 5-3 loss to the Brewers, they committed three errors (only a scorer’s mercy prevented it from being four), Steven Matz allowed five runs in six innings, and an injury-depleted lineup missed numerous chances to cash in.

The Mets (34-28) ended the trip 5-5 after splitting four with the Brewers. They return to Citi Field on Tuesday night to face the Pirates, though it’s unclear whether Collins will be there.

Collins, 67, remained in a Milwaukee hospital overnight for observation after experiencing undisclosed symptoms about 30 minutes before the game. “It was tough,” said Matz, who allowed nine hits. “We’re all concerned about him.”

Yet when it came to the Mets’ most ham-handed effort of the year, Curtis Granderson insisted that fatigue might have played more of a role than the distraction of Collins’ sudden illness. Said Granderson: “I think a 10-day road trip, if anything, was more of the cause to that . . . coming to the end of it.”

Filling in for Collins, bench coach Dick Scott watched the Mets turn routine plays into “a little bit of a disaster,” spotting the Brewers a 5-0 lead.

In the Brewers’ three-run second inning, Keon Broxton laced a run-scoring single off Matz, then swiped second base. With one down and runners on second and third, pitcher Zach Davies laid down a sacrifice bunt that drove in Scooter Gennett from third — and Matz bounced his throw past first baseman James Loney, allowing Broxton to score another run.

The Brewers tacked on two more runs in the fifth, again with the help of a throwing error by Wilmer Flores, who endured a rough day.

Flores could have been charged with another error with two outs in the first when he deflected Ryan Braun’s grounder into shallow left, where Alejandro De Aza scrambled to make a futile throw to second. It got away for a throwing error. Braun was awarded a double and reached third on the play but did not score.

That set the tone for the Mets, who failed to capitalize on the first of the Brewers’ two errors. Despite loading the bases in the first, the Mets came away with nothing, and Davies retired 17 straight batters after escaping that first-inning scare.

The Mets didn’t score until the seventh, when Kelly Johnson led off with a double and came home on De Aza’s forceout. They scored twice more in the eighth on an RBI single by Granderson and a run-scoring double by Yoenis Cespedes.

With two outs, Flores came to the plate as the go-ahead run, only to line out to Braun in leftfield. With that, the Mets missed their last chance to alter their fate on a frustrating day, one that began with Collins forced to trot out a depleted lineup.

Neil Walker’s sore back kept him on the bench and Michael Conforto’s sore wrist limited him to a pinch-hit strikeout. Asdrubal Cabrera was given a rare breather but ended the day in the game, working a walk to extend the eighth-inning rally that eventually fizzled.

It was the final act on a day in which the Mets seemed destined to fall short.

Did Collins’ absence lead to the sloppiness? “The guys are human. It may have,” Scott said. “But on the other side of that, the guys are very professional. As you saw, after early in the game, we could have just phoned the rest of the game in. But they continued to play.’’