Manchester United saw a lot of the ball, but failed to breach the Baggies backline again for the winner. Harry Maguire's late header was tipped over by Sam Johnstone, but it was the home side that enjoyed the best of the chances in the second half.
United are now seven points behind their cross-city rivals, having played a game more.
On that note, let us have a look at the five major talking points from Manchester United's disappointing draw at West Brom.
#1 Manchester United are caught napping early
The hosts displayed infectious intent in the opening seconds of the game. West Brom were at it from the get-gom as they deployed five men in and around the box for long throws.
That spoke volumes of their energy and hunger, while Manchester United were nowhere near that level in the early exchanges.
It took just 83 seconds for the Baggies to expose a lazy Manchester United, as they failed to deal well with a cross that beat David de Gea.
After beating Anthony Martial with some classic overloading down the right flank, Conor Gallagher whipped in a delightful ball into the Manchester United box.
Victor Lindelof, who was well ahead of his marker, stood no chance, as Diagne outmuscled him to head home his maiden Premier League goal.
#2 Bruno Fernandes comes to Manchester United's rescue yet again
West Bromwich Albion did not get carried away with their early opener, and looked to double their lead against Manchester United.
Their stubborn defence also frustrated the visitors. The West Bromwich Albion defence shut down spaces masterfully. As a result, the likes of Marcus Rashford, Luke Shaw and Aaron Wan-Bissaka only received the ball in wide areas.
However, Shaw's brilliant run fashioned an opening for Manchester United on the cusp of half-time. Bruno Fernandes was on hand to strike a sweet volley beyond former Manchester United custodian Sam Johnstone.
Not for the first time, Bruno Fernandes made a goal-contribution since his arrival at the club last year. Only Lionel Messi and Robert Lewandowski (44 each) have been involved in more goals across Europe's top five leagues than Fernandes (38) since the latter's arrival at Manchester United.