Ruud van Nistelrooy may never lead Manchester United in cup competition again, ending up with a 5-2 goalsfest on the right as the interim manager warms to his age.

Two from Casemiro – the opener a real eye-catcher – and Bruno Fernandes and one from Alejandro Garnacho erase Erik ten Hag's dismal tenure and are a fitting gift for Ruben Amorim should he feature in Chelsea's visit on Sunday.

With United pushing to appoint Sporting's head coach as Ten Hag's permanent successor, Van Nistelrooy's term will end once the dispute over reducing the 39-year-old's 30-day notice period reaches a convergence. But if Sunday is too early, Van Nistelrooy will be more than enjoying trying to find a way past Enzo Maresca's fifth place.^­established team.

Van Nistelrooy, dapper in a turtleneck, dark jacket and trousers ensemble, laid out his manifesto for the fight against Leicester, who finished two points and one place – 15th – behind United in the league: “Hopefully a lot of possession and attacks. “That’s what Man United is all about.”

Joshua Zirkzee, one of three substitutions, was able to do just that when he came on, but as is his game, the wrong option was chosen and the Foxes escaped.

But then Casemiro's thunderbolt made Van Nistelrooy scream “ooh” and raise his arms. United's build-up play was decent, Leicester's lack of pressure on them was an oversight. Through an inside left channel, Lisandro Martínez found Alejandro Garnacho, who skipped the infield and moved to Bruno Fernandes. The captain, as he does, let the ball roll and the Brazilian had time to shoot past Danny Ward into the top right corner.

This broke a strangely calm mood, which soon calmed down again for a while. Then a second home strike raised the mood again. This time it was Casemiro who initiated the attack, placing Dalot on the right. He shifted his body towards the ball and delivered a cross that was smashed by Garnacho.

Cut to an anguished Steve Cooper in his technical area, booked for offside for protesting at Dalot – and his desperation almost increased when Fernandes placed a shot from the edge of the box. But the No. 8 instead allowed himself to make a no-look pass and deflected it astray.

United have become experts at shooting themselves in proverbial style since the resignation of Sir Alex Ferguson in May 2013. Nothing changed under their second Dutch coach of the season: Altay Bayindir, United's third new signing, hit a cross, the ball went to Bilal El Khannouss and he hit the left post with a pinball.

The prospect of another United collapse lasted for six minutes – the length of time recorded by Fernandes and Casemiro. El Khannouss tripped over Garnancho and Fernandes hit a deflected free-kick past Ward; Then Casemiro hit the rebound off both Leicester posts – the left first – after his own header from an assist from Rashford.

All was well for United until a mix-up between Casemiro and Dalot and Conor Coady in first-half stoppage time to give Van Nistelrooy something to chew on at the break.

Although the caretaker manager fielded a second number 6 in Manuel Ugarte, the gaps in United's midfield, an unwanted legacy of Ten Hag, remained and Cooper's unit exploited them in several worrying attacks.

Technically, Van Nistelrooy reacted to this and other sloppy moves by United with obvious alarm. The worst thing was that James Justin came in from behind and crossed, a flailing Casemiro blocked it and he ran to safety in the following corner.

There were more to come, often from the boots of the firefly El Khannouss, whose clever wing play had plunged United into chaos. Then finally a clever United attack at half-time that ran through Fernandes-Rashford-Fernandes-Zirkzee-Garnacho and ended with a Zirkzee fingertip warmer for Ward.

And then came United's fifth goal. A Fernandes-Rashford combination caused a brain freeze for Caleb Okoli, which resulted in Fernandes making no mistakes. A dummy brought Ward to the ground and the 30-year-old couldn't have missed the target from just a few centimeters away.

United cruised. Rashford and Martinez came in, Amad Diallo and Noussair Mazraoui were added. Ugarte, United's Red Adair, turned into a schemer, turning and firing an aerial ball to Fernandes on the left. It was a rarity for United: an evening of this kind.