Ruud van Nistelrooy may never lead Manchester United in cup competition again, so ending on the right end of a 5-2 goal-fest as the interim manager can warm him in his dotage.

Two from Casemiro – the opener a screamer – and Bruno Fernandes plus one from Alejandro Garnacho wipes away the dog-days gloom of Erik ten Hag’s tenure, and is an apt gift to Rúben Amorim should he be in place for Chelsea’s visit on Sunday.

With United pushing to appoint Sporting’s head coach as Ten Hag’s permanent successor, Van Nistelrooy’s stint will end as soon as the wrangling over abbreviating the 39‑year‑old’s 30-day notice period reaches rapprochement. Yet if Sunday is too soon, Van Nistelrooy will more than relish trying to plot a way past Enzo Maresca’s fifth-positioned team.

Van Nistelrooy – dapper in an ensemble of turtleneck, dark jacket and trousers – set out his manifesto to take on Leicester, who arrived two points and a place – 15th – below United in the league. “Hopefully lots of possession and attacks,” he said. “That’s what Man United is about.”

Joshua Zirkzee, one of three changes, could do precisely this when roving in but, as is the faultline in his game, the wrong option was chosen, and the visitors escaped.

But then Casemiro’s thunderbolt had Van Nistelrooy “oohing” and raising his arms. United’s buildup was neat, Leicester’s lack of pressure on them a dereliction. Through an inside left channel, Lisandro Martínez found Garnacho, who skipped infield and tapped to Fernandes. The captain, as he cutely does, let the ball roll and the Brazilian had an age to fire past Danny Ward into the top right corner.

This broke a vibe that had been oddly becalmed, a mood that soon again settled for a period. Then a second home strike ratcheted spirits once more. Casemiro ignited the move this time, locating Diogo Dalot along the right. He shifted his body to face the ball and sliced in a cross that was smashed in by Garnacho.

Cut to a pained Steve Cooper in his technical area, who was booked for protests that Dalot was offside – and his despair nearly deepened when Fernandes lined up a shot from the edge of the area. But the No 8 plumped, instead, for a no-look pass and he misdirected.

United, basically since Sir Alex Ferguson retired in May 2013, are experts at shooting themselves in the proverbial. This did not change under their second Dutch manager of the season: Altay Bayindir, the third of United’s fresh personnel, flapped at a cross, the ball went to Bilal El Khannouss, and he scored via a pinball off the left post.

The prospect of another United collapse lasted six minutes – the span in which Fernandes and Casemiro registered. El Khannouss tripped over Garnancho and Fernandes flipped a deflected free-kick in past Ward; then Casemiro touched home the rebound off both Leicester posts – their left one first – after his own header from a Marcus Rashford delivery.

All fine and dandy for United until, in first-half added time, a Casemiro and Dalot mix-up, and Conor Coady bundled in to give Van Nistelrooy something to chew on at the break.

Despite the caretaker fielding a second holding midfielder in Manuel Ugarte, the gaps in the United midfield that are an unwanted Ten Hag legacy remained and Cooper’s unit ran through these in several concerning attacks.

Van Nistelrooy, in the technical area, met these and other sloppy United play with obvious disquiet. The worst featured James Justin getting in behind and crossing, a flailing Casemiro blocking, and, at the ensuing corner, the latter heading to safety.

More came, often from the boots of the firefly El Khannouss whose smart wing play had United in disarray. Then, at last, a slick United raid of the half that ran through Fernandes-Rashford-Fernandes-Zirkzee-Garnacho and ended with a Zirkzee fingertip-warmer for Ward.

And then came United’s fifth. A Fernandes-Rashford combo caused a Caleb Okoli brain freeze that resulted in Fernandes being clean through. A dummy took Ward to ground and the 30-year-old was never going to miss from inches away.

United cruised. Off came Rashford and Martínez, on came Amad Diallo and Noussair Mazraoui. Ugarte, United’s Red Adair, morphed into a schemer pivoting and spraying an aerial ball to Fernandes down the left. It was, for United, a rare thing: this type of night.