The Red Devils called time on the Dutchman's 29-month reign on Monday, leading to the club's popular former striker stepping up from his role as assistant manager while a permanent head coach is recruited.

United are in talks to hire highly-rated Sporting Lisbon boss Ruben Amorim and the club appear to be safe in hands should negotiations drag on judging by Wednesday's win against Leicester.

Van Nistelrooy – who returned to the club in the summer as one of Ten Hag's assistants – celebrated as excitedly as those in the Old Trafford stands when Casemiro opened the scoring with a stunner 15 minutes into his first match in temporary charge.

Alejandro Garnacho struck another before Bilal El Khannouss pulled one back for much-changed Leicester, only for United to hit back through a deflected Bruno Fernandes free-kick and Casemiro's second of the night.

Conor Coady cut the lead in first-half stoppage time, but Fernandes coolly scored his second on an evening when fan favourite Van Nistelrooy's name frequently filled the air.

United had won just one of their eight matches in all competitions since swatting aside Barnsley 7-0 in the previous round of the Carabao Cup and they looked hungry after what Van Nistelrooy called a "hectic" build-up.

The caretaker boss made four alterations against Leicester, whose manager Steve Cooper made nine changes and saw his side fall behind to a fantastic 15th-minute opener.

Fernandes smartly stepped over a Garnacho pass, giving Casemiro space to take a touch and unleash an outstanding strike from nearly 30 yards that crashed in off the underside of the bar.

Van Nistelrooy raised his arms aloft in celebration, looking as stunned as any fan by the quality of the goal.

United's caretaker boss was applauding another in the 28th minute.

Displaying the kind of quality and cutting edge lacking in Sunday's costly loss to West Ham, Diogo Dalot was found by a floated ball down the flank and fizzed a low delivery to the far post for Garnacho to slam home.

Leicester's loud away end had been silenced but they found their voices again in the 33rd minute.

Altay Bayindir pushed away a cross and, despite setting himself for an attempt, could not stop a El Khannouss strike whizzing in off the far post in front of the Stretford End.

But hope of a comeback was short lived, thanks in part to the Leicester goalscorer's clumsy foul on Garnacho just outside the box.

Fernandes stepped up to a free-kick that took a huge deflection off James Justin, wrongfooting Danny Ward and flying into the back of the net.

United's third had arrived in the 36th minute and their fourth followed just three minutes later.

Casemiro rose to meet Marcus Rashford's cross with a header that bounced off both posts, then reacted quickest to lash in the loose ball.

The hat-trick chasing midfielder narrowly headed wide from a free-kick, unaware he was offside, before the Foxes reduced the deficit as Dalot inadvertently directed a stoppage-time free-kick into the path of Coady.

Leicester returned for the second half with renewed purpose but United eventually wore them down.

Ward denied Joshua Zirkzee and could do little when Fernandes latched onto Caleb Okoli's under-hit back pass to round the goalkeeper and then finish in the 59th minute.

Van Nistelrooy applauded when substitute Amad Diallo acrobatically struck an overhead kick narrowly wide, with Boubakary Soumare seeing a slightly deflected drive tipped onto the bar by Bayindir at the other end.

No more goals were to follow despite United's best efforts to add gloss to a win that should put a pep into their step heading into Sunday's Premier League home clash against Chelsea.

Man United begin post-Erik ten Hag era with victory against Leicester in Carabao Cup