After falling down the pecking order at Old Trafford, the 26-year-old moved to Sevilla on loan in January until the end of the season.
However, the Frenchman struggled with the La Liga outfit, scoring only once in 12 appearances across all competitions.
Sevilla have now rejected the chance turn his temporary deal into a permanent transfer and the forward will return to Old Trafford this summer.
Speaking to reporters, via The Sun, Castro said: "Anthony Martial will return to Man United. We spent important money on him but it didn't work.
"Also, he had injuries and we're not going to buy him on a permanent deal".
It remains to be seen whether Martial, who has two years remaining on his £240,000-per-week contract, will feature in Erik ten Hag's plans in the 2022-23 campaign. body check tags ::