A point was the least Brentford deserved after dominating throughout and hitting the woodwork on four occasions. However, it looked like Thomas Frank's men were to be hit by a sucker punch when Mount struck to ease the pain of his injury-plagued first season with the Red Devils.
But United failed to hold out, inflicting another blow to their chances of Champions League qualification, as Ivan Toney teed up Ajer to smash home.
Victories for Aston Villa and Tottenham earlier in the day upped the pressure on United to respond in the battle for Champions League football next season. A draw leaves Erik ten Hag's men 11 points off the top four and eight adrift of fifth-placed Tottenham.
But it was the manner of the performance that was just as concerning for Ten Hag on the back of a thrilling 4-3 FA Cup quarter-final win over Liverpool in their last outing before the international break.
It was back to the drawing board for the Dutchman as his side were comprehensively outplayed by a Brentford side that have won just twice in 16 Premier League games.
Brentford famously beat United 4-0 at home last season in Ten Hag's second game in charge with a fraction of the chances they enjoyed this time around. Toney was fresh from scoring his first England goal in midweek and was keen to further his case to be Harry Kane's deputy at Euro 2024.
The Bees captain was just one of four Brentford players to hit the woodwork as they peppered the United goal with 31 shots. Toney scuffed his effort that came back off the post midway through the first half after being played in by Yoane Wissa's fine through ball.
Mathias Jorgensen then headed off the top of the crossbar and Wissa clipped the outside of the post either side of the break. Rasmus Hojlund had
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