With all eyes on under-pressure manager Jose Mourinho, United conceded twice in the early stages at home on Saturday, Kenedy and Yoshinori Muto scoring the goals.
Kenedy netted in the seventh minute, while Muto's strike came with nine minutes and 14 seconds on the clock.
It was the first time United had conceded twice in the opening 10 minutes of a Premier League game at Old Trafford, as Mourinho's difficult season looked to be taking another turn for the worse.
United were reeling, and a stunned Old Trafford nearly witnessed a third for Newcastle before the break when Muto's six-yard header was superbly clawed away by David de Gea in the United goal. The team were booed off as they left the pitch at half time.
But just when all seemed lost, United staged an unlikely fightback, Juan Mata halving the arrears and restoring hope around Old Trafford with a well-placed free-kick after 70 minutes.
And six minutes later they were level, Anthony Martial collecting the ball inside the box and riffling past Martin Dubravka in the Newcastle goal.
Then, as 90 minutes ticked over and with the game seemingly heading for a draw, under-fire substitute Alexis Sanchez rose at the far post to nod into the bottom corner and send Old Trafford into raptures.