The Italian knows the foundation for any team serious about challenging for honours -- primarily a domestic title -- is a player who will score at least 20 league goals a season. But by allowing Lukaku to leave Everton for Manchester United, Chelsea have let a proven, prolific Premier League goalscorer slip through their grasp.

Conte already has one of those, of course, at Stamford Bridge, but Diego Costa has made it clear he sees his future away from the club after receiving a text message from his manager telling him he no longer fits into his plans at Chelsea.

At the end of last season, when Costa scored 20 league goals to propel the London club to the title, Conte's decision to be blunt with the Spain international by telling him he was yesterday's man was bold, but it may now prove to be reckless in the wake of Lukaku's move to Old Trafford.

Conte believed he was going to get the Belgian as Costa's replacement -- a younger, less temperamental version of the former Atletico Madrid forward -- but those plans are now in tatters and Chelsea have a problem.

United and Jose Mourinho are celebrating Lukaku's capture because they know that top class centre-forwards are becoming an increasingly rare commodity.

Wayne Rooney, who ended his 13-year stint at Old Trafford by returning to Everton over the weekend, was no longer able to provide the goalscoring consistency of his younger years at United, while Zlatan Ibrahimovic, at 35, was never going to be a long-term solution, even before the cruciate ligament injury which ended his season last April.

Mourinho and United have been searching the market for a new centre-forward since last summer and Lukaku was always high on the list, but the common belief within the game was that he was destined for Chelsea.

So United looked instead at Torino's Andrea Belotti and Real Madrid's Alvaro Morata, having reluctantly accepted there was no realistic prospect of prising Harry Kane from Tottenham or Robert Lewandowski from Bayern Munich.

As for re-signing Cristiano Ronaldo, United have been burned once too often by that particular fantasy to even consider getting excited when the Portuguese threatened to leave Real Madrid for a brief period of time last month.

There were too many doubts over Belotti for United to make him their No.1 target, but a move for Morata, Ronaldo's Real Madrid teammate, was well underway until executive vice-chairman Ed Woodward's persistence over Lukaku paid off earlier this month when it became clear Chelsea had not pushed quite hard enough to get their man.

The rest is history. United arrived with a late run and sealed a deal, leaving Chelsea with red faces and complaints that Lukaku would have signed for them but for the influence of his agent, Mino Raiola, who had already helped arrange three other transfers to United in the past 12 months.

Whatever Lukaku's reasons for choosing United over Chelsea, the reality is that he will be wearing a red shirt next season rather than a blue one and the sense of rejection at Stamford Bridge must evaporate quickly if they are to find a different solution.

Can Conte patch it up with Costa? Does he even want to do so?

That relationship appears to have been broken beyond repair by Conte's farewell message to the player, so what, and who, now?

Chelsea retain a firm interest in Alexis Sanchez, but Arsenal are determined not to sell the Chilean. If he does leave the Emirates, a move abroad appears to be the only outcome Arsenal would even consider.

Diego Costa & Romelu Lukaku Chris Brunskill/Getty Images

So Chelsea find themselves facing the same puzzle as United before their Lukaku coup -- where can they find a striker capable of hitting those 20 goals a season?

Not since United won the title in 2008-09 has a team finished top without a 20-goal-a-season man.

Back then, Ronaldo topped their scoring charts with 18 league goals -- Rooney hit 12 that year -- but United had goals throughout that team and the combination of Ronaldo and Rooney won them plenty of games on the way to the title.

Chelsea must either spread the load of goals, squeezing more out of Eden Hazard, Pedro and Michy Batshuayi, or find somebody to replace Costa and fill the void that has been left by Lukaku's decision to choose United.

Just like United, they have also monitored Belotti and Morata. Just like United, Chelsea believed Lukaku was the best option.

So they are now likely to have to settle for second best, at the same time as losing the striker who hit 20 crucial goals in the league last season.

It is a problem Chelsea have to solve because they will look around at their top six rivals and see Kane at Spurs, Sanchez at Arsenal, Sergio Aguero at Manchester City, Sadio Mane and Roberto Firmino at Liverpool and Lukaku at United.

All of their likeliest challengers for the title have reliable goalscorers, but Chelsea and Conte know their best striker is heading for the exit door with no replacement in sight.

If it stays that way, their title defence will be over before it even begins.