Now, instead of being able to cut the Londoners' lead to three points, defeat would leave United 11 points adrift having played a game more.
Ten Hag has so far talked down United's unlikely title challenge, but the improvement in his team has been evident on the back of nine straight wins before Palace, including a 2-1 victory against second-place Manchester City last weekend.
United remains the only team to have beaten Arsenal in the league this season and a repeat performance at the Emirates Stadium would have been a major statement of intent.
It still could be, but the draw with Palace will feel like a missed opportunity after Bruno Fernandes had opened the scoring in the 43rd.
United moved up from fourth to third in the league — level on points with Manchester City.
While Ten Hag and his United players were left frustrated, Leeds can look forward to an FA Cup fourth round match with Boreham Wood or Accrington Stanley after a 5-2 rout of Cardiff in their third round replay.
As if a trip to high-flying Arsenal on Sunday isn't daunting enough, United will be without one of its most influential players.
Casemiro picked up a fifth yellow card of the season against Palace, meaning the midfielder will be suspended for Sunday's game.
“He’s a really important player for us and a reason we’re in this position. But last time we beat Arsenal without Casemiro. We need to do it again," said Ten Hag.
Casemiro has been one of United's best players this season, losing just two of the 18 games he's started since joining from Real Madrid in the summer.
FIRST PREMIER LEAGUE START
American defender Chris Richards made his first Premier League start for Palace after four substitute appearances. The 22-year, who partnered with Marc Guéhi in central defense, joined Palace last summer from Hoffenheim. Richards was sidelined for four months until returning Dec. 26.
Leeds put its league struggles behind it with a morale-boosting win in the FA Cup.
The 5-2 victory against second-tier Cardiff ended a seven-game winless streak in all competitions.
It took a 90th-minute goal from Sonny Perkins to salvage a 2-2 draw in the original game but the replay was a much more one-sided affair.
Leeds led 3-0 after 36 minutes with two goals from Wilfried Gnonto and another from Rodrigo. Gnonto gave Leeds a flying start with a stunning volley inside the opening minute and fired home his second after Rodrigo had struck his 11th goal of the season.
Patrick Bamford struck twice in the second half, while Callum Robinson pulled two back for Cardiff.
Premier League survival remains Leeds' priority as it sits just two points clear of the relegation zone.
“I felt very strongly before the game that we were ready for a strong performance. I told the team that. This was obviously an important moment for us, but it’s just another moment in the development we’re trying to create," Leeds manager Jesse Marsch said.
“An important moment to gather momentum, but hard to say validation because we still have so much to do.”
More AP soccer: https://apnews.com/hub/soccer and https://twitter.com/AP_Sports
James Robson is at https://twitter.com/jamesalanrobson