Veteran striker Alex Revell was coolness personified from the spot, sending Sergio Romero the wrong way as he drove the ball low to the keeper’s left.
“Sacked in the morning,” the home fans sang to Mourinho, who had stood with hands deep in his trouser pockets through the whole sorry episode.
Mourinho had his men doing some light training in the car park of the nearby four-star Daventry Court Hotel, having stayed there on the eve of the game.
The manager, who came up with the dig “parking the bus” to describe a defensive opponent, was no doubt hoping the League One team would not put up the barriers in a bid to shut out and frustrate their star-studded opponents.
Despite making nine changes from the side who lost 3-1 at Watford on Sunday – only skipper Rooney and centre-back Chris Smalling stayed in – Mourinho still picked a strong team packed full of internationals.
His bench was even stronger, including Zlatan Ibrahimovic and Marcus Rashford.
Rooney scuffed a shot wide early on and his struggles continued in the 31st minute when he was adjudged to be offside after tapping in a rebound when Fosu-Mensah’s header struck the bar.