Rashford's curling free-kick in the ninth minute set Barcelona on their way to a 2-0 victory against Real Madrid at Camp Nou that secured back-to-back La Liga titles. He has scored 14 goals and provided 14 assists in 46 appearances since joining on loan from Manchester United in January, and is the first Englishman to win La Liga in 41 years.
Koeman, who made 193 appearances for Barcelona between 1989 and 1995 and managed the club from 2020 to 2021, offered an unambiguous verdict to AS on the transfer question.
"If Barcelona let him return to Manchester United after this loan, I think they will regret it immensely. Because €30million in the current market for a player with these characteristics, these numbers, this experience — that's a rip-off."
He was emphatic about the specific qualities Rashford had displayed against Real Madrid.
"Rashford hurts teams. Madrid looked terrified every time he turned and ran. Against Real Madrid, he completely destroyed them on the counter-attack. The speed, the aggression, the directness, the confidence — Madrid couldn't handle him. Every time Barcelona advanced, he was the danger."
Koeman said the hesitation within the club over the fee was baffling given what they had witnessed.
"He scores a free-kick in El Clásico, stretches the entire defensive line, creates numerical advantages, presses, gets in behind the defence — and yet there are people within the club who hesitate to pay €30million? That seems insane to me."
Barcelona's reluctance has centred on their ongoing financial constraints under La Liga's economic control rules. Manchester United want a permanent sale and will not agree to a second loan. Rashford himself has said he would like to stay, but "is not a magician." The decision is Barcelona's to make — and Koeman's verdict on the cost is the clearest public endorsement yet that they should make it.