Diego Costa will make his Premier League bow for Chelsea against Burnley this evening, but part of Jose Mourinho will be still harbour major regrets over his failed pursuit of Manchester United striker Wayne Rooney last summer.
Mourinho's admiration for Rooney is no secret. The Portuguese coach recently insisted he tried to bring Costa to Stamford Bridge from Atletico Madrid this time last year, but that's in direct contrast to what he admitted at the time.
The question 'is it Rooney or bust?' was put to Mourinho last July, to which the Chelsea boss responded with a firm 'Yes'.
United were not prepared to do business
In the end, he settled on veteran striker Samuel Eto'o, who was handed a one-year deal. This indicated that Mourinho was still optimistic he would land Rooney either in January or the following summer.
However, United were not prepared to sell their star player to a direct Premier League rival - or any other club for that matter - despite Sir Alex Ferguson's claims that the England international had submitted a transfer request at the end of the 2012/13 campaign.
David Moyes worked tirelessly throughout his brief, ill-fated reign at Old Trafford to make Rooney feel like Manchester United's main man again - and it worked.
Rooney: Now United for life
Moyes revealed in a recent interview with the Daily Mail that, had he avoided the axe at Old Trafford, he would have handed Rooney the captain's armband. Louis van Gaal made the same decision earlier this month, and the 28-year-old now appears almost certain to spend the rest of his career with the Red Devils, becoming the club's all-time top scorer in the process.
Rooney would have been a phenomenal signing for Chelsea. The Blues would have almost certainly won their first Premier League title since 2010 back in May had they secured the £300,000-a-week star's signature.
Watch: Wayne Rooney's top ten goals
Mourinho would prefer Rooney to Costa
This is a player who guarantees goals and assists. Rest assured, had there been the slightest hint that United were prepared to sell this summer, Mourinho would have been at the front of the queue, Abramovich-signed cheque in hand, without a thought or care for Diego Costa.
Costa did remarkably well for Atletico Madrid last season, scoring 36 goals in 52 appearances, but whether he can replicate the same clinical form at Stamford Bridge remains to be seen.
After all, this is a striker who had never scored more than 10 league goals in any of his eight seasons as a professional before last term's heroics at the Vicente Calderon.
Costa: Expensive gamble
Costa, therefore, represents an expensive gamble. While his pre-season form has been impressive, Chelsea fans who watched his dismal performances for Spain at the World Cup finals in his native Brazil have good reason to still feel a little apprehensive about the transfer.
This is also a player who struggles to keep his cool on the pitch. If he emerges from his first full season at Stamford Bridge without one or two red cards to his name it'll be nothing short of a miracle.
Rooney used to be similarly hot-headed, but has worked hard on the ability to control his temperament during testing circumstances.
The United skipper is already off the mark for the 2014/15 campaign and could easily finish the season with 30 goals to his name. Should that happen, missing out on Rooney will remain one of the biggest regrets of Mourinho's managerial career.
He might have Costa, but he'll be wishing it was Rooney leading Chelsea's fearsome attack instead.