Newcastle United punished for defensive errors at the London Stadium
The club's three-game unbeaten run was brought to an end by West Ham United, who won 2-0 thanks to goals from Declan Rice and Mark Noble.
Rice lost Fabian Schar, his marker, to head the home side into the lead, while Noble's strike came from the penalty spot after a mistake from Florian Lejeune.
Miguel Almiron, so influential in the home wins over Burnley and Huddersfield Town, didn't see enough of the ball in the fixture, which looked like it was a game too far for Benitez's side.
The result left the club 14th in the Premier League and six points above the relegation zone.
"I'm disappointed," said manager Rafa Benitez, who lost midfielder Sean Longstaff to injury at the break.
"Hopefully, we will learn from this in the future and not make these same mistakes in the next nine games.
"We have nine games to play, and still we are in a relegation battle so we have to be sure that we approach them like a final. We cannot be thinking we are safe, no chance."
West Ham legend Billy Bonds had a stand named at the London Stadium named after him before the game.
It wasn't long before Newcastle were behind. Robert Snodgrass swung in a swung in a seventh-minute corner, and Rice, watched by England assistant manager Steve Holland, lost Schar to head home.
Javier Manquillo, again preferred to DeAndre Yedlin at right-back, clashed heads with Aaron Cresswell soon after the goal. A bandaged Manquillo was able to continue after treatment.
Newcastle, so composed in possession in recent weeks, struggled to keep the ball for any length of time in the first 25 minutes.
West Ham were quicker, sharper and more incisive whenever they had the ball. They came back again and again, and Schar and Longstaff, making an 11th successive start, were booked for crude challenges.
Newcastle were pedestrian compared to their hosts in the first half, and Almiron never got into his stride.
And it got worse before the break. Noble scored from the spot after Lejeune dived in and brought Javier Hernandez down in the box.
Benitez – who rarely makes half-time substitutions – replaced Longstaff with Mohamed Diame for the second half after the 21-year-old suffered a knee injury.
Diame set about disrupting West Ham's midfield after his introduction, though that proved difficult. Pellegrini's team continued to dominate the ball, and Benitez made a tactical change midway through the half.
Paul Dummett replaced Schar, and Benitez went to a four-man defence with Matt Ritchie being switched to right wing.
The changes steadied Newcastle, but there was to be no late comeback, despite a late rally. Angelo Ogbonna blocked a late shot from Rondon, who had got on the end of a cross from substitute Christian Atsu.
Rondon had another effort tipped on the the post by Lukasz Fabianski.
WEST HAM UNITED: Fabianski, Cresswell (Masuaku, 34), Fredericks, Ogbonna, Diop, Rice, Lanzini, Noble (Obiang, 80), Snodgrass, Chicharito (Arnautovic, 70), Anderson. Subs not used: Adrian, Nasri, Perez, Antonio.
NEWCASTLE UNITED: Dubravka; Manquillo, Schar (Dummett, 65), Lascelles, Lejeune, Ritchie; Perez, Longstaff (Diame, 46), Hayden (Atsu, 79), Almiron; Rondon. Subs not used: Darlow, Yedlin, Ki, Muto.
Goals: Rice 7, Noble 42 (pen)
Bookings: Noble 26, Schar 37, Longstaff 40, Masuaku 67, Hayden 67, Dubravka 85, Diop 89
Chris Kavanagh (Manchester)