Goals from Burnley's Chris Wood and Jay Rodriguez inflicted an embarrassing 2-0 defeat on Manchester United at Old Trafford, a loss that will surely bring Ole Gunnar Solskjaer's position into question once again.
It was Solskjaer's first home league match as United manager without Marcus Rashford - who is set for a long absence with a back injury - and his absence was telling, as the Red Devils showed little cutting edge in the final third against a Burnley side that was typically set up to frustrate.
Although United dominated much of the first half, the chances it created were wasted and Burnley was far more clinical, going into the break with a one-goal lead thanks to Wood despite having just three shots to the hosts' 11.
United was arguably even worse after the interval. The opportunities it had been crafting dried up and a phenomenal strike from Rodriguez ultimately gave it too much to do, meaning fourth-placed Chelsea remains six points clear of Solskjaer's men.
United was most threatening down the right in the first half, and Wan-Bissaka - routinely given freedom by Burnley - picked out Anthony Martial in the 15th minute, but the Frenchman could not get enough contact with the ball.
Wan-Bissaka again caused trouble in the 28th minute as his low delivery found Juan Mata, only for the midfielder to completely miss his kick as the ball struck his standing leg.
Two more chances soon went begging, with Daniel James' header forcing a save and Martial spurning a decent opening, and Burnley capitalised just before half-time - Wood steering past David de Gea at the near post after Ben Mee's flick-on.
Solskjaer introduced Mason Greenwood for the abject Andreas Pereira at half-time, though it made little difference and United soon found themselves 2-0 down as Rodriguez peeled away from Harry Maguire and lashed a scorching effort in off the crossbar from just inside the box.
United thought Charlie Taylor should have been shown a red card after he hauled Greenwood down on the edge of the box when the substitute would have only had Nick Pope to beat, but referee Jon Moss and the VAR did not agree.
Burnley had little trouble holding on to move to within just four points of its host, whose late goal by Luke Shaw was chalked off for a foul on Jeff Hendrick.
What does it mean? Reinforcements a necessity for United
Although it has been fairly obvious for a while, it now seems absolutely vital that United makes some meaningful purchases in the January transfer window.
Its lack of depth was plain to see without Rashford, and those in midfield were generally hopeless from a creative perspective. Without reinforcements, Solskjaer needs a miracle to get this bunch into the Champions League.
Wood gets the better of United
There are not many strikers in the Premier League more predictable than Wood in some senses - but that does not mean you are guaranteed to stop him. The burly frontman kept United's defence occupied throughout and he proved very effective, as highlighted by his goal and then his assist for Rodriguez.
Pereira not the answer
While there are not many United players who could have avoided criticism on Wednesday, Pereira was - once again - simply not up to scratch. The midfielder saw a lot of the ball but did precious little with it, his four shots causing Pope no issues. He was unsurprisingly withdrawn at half-time.
It is back to the FA Cup for these two at the weekend. United go to either Watford or Tranmere Rovers on Sunday, with Burnley hosting fellow Premier League side Norwich City the day before.