Man Utd's slow start angers Ten Hag in draw with Leeds
Leeds sacked Jesse Marsch on Monday following a run of seven league games without a win.
Caretaker manager Michael Skubala had just one 20-minute training session to prepare for the trip across The Pennines, but the visitors had been on course for a first victory over United since 2010.
Wilfried Gnonto gave Leeds the perfect start after just a minute and Raphael Varane's own goal doubled their advantage early in the second half.
Marcus Rashford's 20th goal of the season sparked the United fightback before Jadon Sancho's first goal since September stretched the Red Devils unbeaten run at home to 16 games.
"You can say we won a point because when you are in a derby and come back from 2-0 down it is good," said Ten Hag after the first of two Premier League clashes in five days between the sides.
"But on the other hand when you start a derby like we did it's unacceptable. You have to be ready to fight and we weren't ready for the battle."
A point leaves third-placed United still seven points adrift of leaders Arsenal, having played two games more.
The draw is more valuable to Leeds, who edge one point clear of the relegation zone.
Casemiro missed
United badly missed the presence of the suspended Casemiro in midfield, which allowed Leeds to counter-attack at will.
Gnonto has been a consistent bright spark of an otherwise sad season for Leeds and exploded the game into life after just 60 seconds as he found the bottom corner for his fourth goal in seven games.
United should have been level by half-time as Alejandro Garnacho fired just wide and then saw a shot cleared off the line by Max Woeber after the Argentine had rounded Illan Meslier.
However, the home side were slow out of the blocks after the break and were punished.
Gnonto was at the heart of the move again as he released Crysencio Summerville down the left and his cross was turned into his own net by Raphael Varane.
"I think it instills belief in the players that they are good enough at this level," said Skubala, who remains unsure if he will be in charge for the visit of United to Elland Road on Sunday.
"They know that and we know that and we just need to go after every game between now and the end of the season."
Ten Hag turned to his bench for solutions and found them in Sancho and Facundo Pellistri to replace the ineffectual Wout Weghorst and Garnacho.
Weghorst's withdrawal put Rashford through the middle and within minutes he rose highest to meet Diogo Dalot's cross for his 12th goal in 14 games since the World Cup.
Leeds were inches away from restoring their two-goal lead almost immediately when Brenden Aaronson's free-kick came back off the post.
But their advantage was soon wiped out completely 20 minutes from time.
Sancho's fitness struggles have sidelined him for most of the season, but his return to action has been a timely boost with United still involved in four competitions and short on attacking options.
The England international collected Luke Shaw's deflected cross and his low strike caught out Meslier low to his left.
"He's all the way back," added Ten Hag. "We know he's a magnificent player.
"I think consistently he can have a big impact. But he has to work hard. It will motivate him to get more."