It was all looking so good for José Mourinho. Having seen his Manchester United side come back from conceding the opening goal to lead 2-1 through two strikes from Juan Mata, a last minute lapse of concentration in defence allowed Harry Maguire to snatch a point with the final kick of the match.

United had appeared to be coasting when the Leicester substitute Daniel Amartey was sent off for a second booking 16 minutes from time, with Jesse Lingard wasting a golden opportunity to seal the points when he struck a post. But just as it seemed Mourinho’s side would at least keep pace with Manchester City after their record-breaking 17th win in a row, Maguire’s volley at the back post ensured they now face a 13-point gap with half of the campaign completed.

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This ground has bad memories for the Portuguese after the 2-1 defeat to Claudio Ranieri’s title-chasers two years ago proved to be the last act in his second spell as Chelsea manager. There have been shades of the manager who lost control so quickly at Stamford Bridge about Mourinho’s demeanour this season and his reaction to the dropping of two vital points was an indication that the pressure being applied by Pep Guardiola’s side is taking its toll.

It was unclear exactly who he was singling out for the “childish decisions” that he said cost his side so dear but the finger could probably be pointed at Chris Smalling, who was left bamboozled by Riyad Mahrez’s quick feet for Jamie Vardy’s opening goal and was then at fault for allowing Maguire to pounce at the death. Yet the evening could have ended so differently had United been more clinical in front of goal, with Anthony Martial also spurning a great chance just after the break.

In a week in which they had also crashed out of the League Cup to the Championship side Bristol City, the return of Paul Pogba to Premier League action after his suspension at least gave Mourinho a reason to be cheerful, with the Frenchman captaining the side in the absence of Antonio Valencia – one of only three players to keep his place from the defeat at Ashton Gate.

After four straight victories under Claude Puel, Leicester’s bubble had been burst by the surprise home defeat to Crystal Palace last Saturday and the penalty shootout loss to City. Their manager was criticised for starting that game with Vardy and Mahrez on the bench when the competition perhaps represented their best chance of silverware, and Puel reverted to his first choice side for this match.

But it was United who took the game to their opponents in the opening exchanges. Smalling should have done better with a headed chance from Juan Mata’s centre in the third minute, while Lingard and Pogba tested Kasper Schmeichel with early efforts. But after the initial burst, the visitors began to look laboured going forward, with Romelu Lukaku again struggling to make an impact.

Having created virtually nothing beforehand, however, it was a classic piece of counterattacking that helped Leicester go ahead. Lingering around the edge of his own penalty area, the England forward pounced on a clearance and laid the ball into the path of Wilfred Ndidi, whose first-time pass sailed over the United defence and into the path of the onrushing Mahrez. The Algerian was forced to check his run slightly but showed great awareness to pause briefly before laying the ball to Vardy on a plate.

Their joy was short-lived, though, as a canny left-footed finish from just inside the area from Mata after a clever layoff from Lingard meant United headed into the break on level terms. Despite that setback, Leicester persisted with the tactic of sitting deep and attempting to strike out on the break.

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The strategy almost paid dividends seven minutes into the second half when more brilliance from Mahrez saw him reach the byline, only for Demarai Gray to miss the ball completely when it was played across the face of goal, with David de Gea saving the follow-up from Christian Fuchs.

Martial partially redeemed himself for the miss when he won a free-kick just outside the penalty area and Mata stepped up to curl home his second goal, although Schmeichel will feel he could have done more to prevent the ball finding the net having presented his opponent with a yawning gap to aim at.

Amartey’s second booking, for a reckless bodycheck on Marcus Rashford less than 20 minutes after he had replaced Danny Simpson, seemed to have ended the game as a contest.

But in the final seconds of injury time, everything fell apart for United. Marc Albrighton’s excellent deep cross picked out the unmarked Maguire at the back post and, having fluffed a great chance earlier, this time the England defender made no mistake, to send United into the Christmas period knowing their hopes of a 21st title are virtually over already.