Chelsea are regretting the sale of midfielder Kevin de Bruyne just 13 months on from the departure of the Belgian youngster, according to Wolfsburg general manager Klaus Allofs.
De Bruyne has scored 14 goals in 46 appearances for the Bundesliga club since moving from Stamford Bridge during the 2014 January transfer window for a reported fee of £16.7m as one of two major departures from west London amid Juan Mata's move to Manchester United.
The Belgium international's form has already seen him linked with a move away from Wolfsburg, whose general manager, Allofs, says Chelsea are lamenting the departure of the 23-year-old.
"At Chelsea they watch with appreciation as Kevin has developed," he told Kicker. "They've seen it confirmed why they were reluctant to let him leave."
United are reportedly among the clubs monitoring De Bruyne since his impressive performances at the World Cup and amid an encouraging start to this season that has seen him net 11 goals in all competitions.
As a result of that form, Wolfsburg are second in the Bundesliga, eight points behind leaders Bayern Munich, while they have an eight-point lead over fifth place Borussia Monchengladbach in the race for a Champions League spot.
Despite their safe position inside the top four, Klaus says only a place in next season's top tier European club competition will secure the future of De Bruyne but warns rival clubs their star player comes with a hefty price tag.
"Kevin will be leaving if we do not qualify for the Champions League come the end of this season or afterwards," he said of the former Chelsea man. "All those other clubs are not blind to see.
"Anyway, they would definitely have to dig very deep into their pockets. That's why most of them are definitely without a chance to sign him."