The Portuguese midfielder was playing for third-tier side Real Sport Clube in a warm-up game against top-flight Estrela da Amadora when the incident occurred.
"To the family, his friends, and the affiliated club Real SC SAD, we address our deepest condolences at this tragic time for Portuguese football," the Lisbon FA said.
Castro joined West Brom's academy in 2021 and made his senior debut in a League Cup match against Arsenal. He later had loan spells at Burton Albion, Notts County and Gateshead before leaving the Championship club in 2023.
West Brom said they would honour Castro ahead of Sunday's match against Burnley.
"Albion will pay tribute to Quevin on Sunday, with a period of applause in the 25th minute of the fixture against Burnley. James Morrison’s men will also wear black armbands," the club said in a statement.
Real Sport Clube and Estrela da Amadora also expressed their condolences on Instagram after the match was postponed.