Manchester United need to match the ambition of their rivals and invest a record-breaking £100 million ($128m) in Napoli defender Kalidou Koulibaly, says David May.The Red Devils remain in the market for additions at centre-half following a frustrating summer of failed recruitment in 2018.Jose Mourinho had been eager to reinforce that area of his squad, and ultimately paid the price for struggles at the back as a difficult start to the season cost him his job. Article continues below Editors’ Picks ‘There is no creativity’ – Can Solskjaer get Man Utd scoring freely again? ‘Everyone legged it on to the pitch!’ – How Foden went from Man City superfan to future superstar Emery out of jail – for now – as brilliant Pepe papers over Arsenal’s cracks What is Manchester United’s ownership situation and how would Kevin Glazer’s sale of shares affect the club? Ole Gunnar Solskjaer is now at the helm and, having made a faultless opening to his reign, is being urged to spend big in the January window.A fellow 1999 Treble winner feels the United boss should be demanding major investment, with there no reason why considerable funds should not be used to top the £75m ($96m) Liverpool paid for Virgil van Dijk and make Koulibaly the most expensive defender in world football.Former Red Devils star May told 888sport: “If you look at United over the last God knows how many years they’ve always gone out and broke transfer records if need be.“They did with Rio [Ferdinand], [Angel] Di Maria, [Paul] Pogba, [Romelu] Lukaku. They allegedly pay [Alexis] Sanchez half a million a week.“If you want the best players then you have to pay top money for them. City do it.“Liverpool did it with Van Dijk and just look at the transformation.“They spent £70 million on a goalkeeper, which is a hell of a lot of money but what that gets you back is guaranteed Champions League football and the possibility of winning the Premier League.“The amount of money United earn then to spend £100m on a defender….so what?“Real Madrid will spend it. Barca will. Chelsea and City will.“If it guarantees you a much better defence then go ahead and spend it.“If Koulibaly becomes available – and there are rumours that he is interested with a fee of £90m – then I’m sure Ole would say he’d be a great signing for Man Utd.”United, who have enjoyed five successive victories across all competitions, will be back in action on Sunday when they take in a trip to Tottenham.Another success at Wembley Stadium would allow Solskjaer’s side to take another position step towards the Premier League’s top four, with there still a six-point gap to be bridged at present.Read the full 888sport interview with David May, including his thoughts on Paul Pogba and United’s transfer window prospects.