Leading El Paso 7-0 on May 18, the 51s appeared poised to win their season-high seventh straight game and run their record to a season-high 21 games over .500 at 32-11.
However, the pesky Chihauhaus erupted for seven runs in the sixth inning en route to dealing Las Vegas a stunning 10-9 defeat.
The 51s have been on a slide since, losing seven of their last eight games in their worst stretch of the year.
Las Vegas, which still has the best record in Triple A at 32-18, suffered its season-high fifth straight loss Sunday in a 10-1 defeat to Sacramento (30-20) at Cashman Field.
Afterward, fiery 51s manager Wally Backman questioned his team’s effort.
“You’ve got to play the game with intensity. We’re not doing that right now. We’re just going through the motions,” he said. “When you’re in first place, you’ve got to be ready to play every day because everybody wants to beat you.
“What I’m seeing is guys are not prepared properly and they’re not going about their business the way they’re supposed to go about it right now.”
Las Vegas has lost three of its top pitchers to the parent New York Mets the past two weeks in starters Jacob deGrom (4-0, 2.58 ERA) and Rafael Montero (4-1, 3.67) and closer Vic Black (seven saves, 1.45).
Reliever Josh Edgin also was summoned by the Mets earlier this month, along with utility man Eric Campbell (.355) and shortstop Wilmer Flores (.307), but Backman said that’s no excuse for his team’s recent skid.
“We lost guys last year and we played good baseball,” he said. “The way we played this last week is the worst baseball I’ve seen this club play in the two years that I’ve been here.
“As tough as it is in New York for the Mets and the way they’re struggling, it should give these guys more incentive to play the game harder and they’re not doing that.”
A day after the 51s were shut out for seven innings by Dan Straily in a 5-2 loss, River Cats right-hander Josh Lindblom limited Las Vegas to a run on five hits in 5 2/3 innings, with eight strikeouts and no walks.
Allan Dykstra hit a solo homer in the sixth and went 3-for-4 to lead the listless 51s, who have mustered only 15 runs in their five-game skid and eight in three straight losses to Sacramento (30-20).
“When you try to go through the motions, you’re going to get the (crap) beat out of you, and that’s exactly what Sacramento has done to us,” Backman said. “I’ve already told (the team) what I thought. I’m pissed. Things need to change.
“You’re here for a reason, and that’s to get better and try to get yourself to the big leagues. The way that everybody, as a group, is playing right now, they’re not going anywhere.”
Giancarlo Alvarado (1-1) struggled in his third start of the season for the 51s, allowing six runs on nine hits in five innings. The veteran right-hander had one strikeout, two walks, and hit three batters.
The River Cats, the hottest team in the Pacific Coast League with five straight wins, scored four in the second off Alvarado, who surrendered a two-run homer to Ryan Ortiz in the fifth.
“Everybody has to step up,” Alvarado said. “We have to minimize mistakes. I didn’t do that today.”
Third baseman Zach Lutz went 0-for-4 as his 15-game hitting streak ended for Las Vegas, which will try to avoid a sweep in today’s 12:05 p.m. series finale at Cashman Field.
“We’re just going through a bad stretch right now. It happens every single year. Every team goes through it,” Lutz said. “We’ve just got to keep battling and working hard. A couple guys will start swinging the bats, that carries over to other guys on the team and then we start going again.
“Right now, nothing’s really going our way. But it’ll turn around, for sure.”
Contact reporter Todd Dewey at email@example.com or 702-383-0354. Follow him on Twitter: @tdewey33.