The 23-year-old collapsed in the ring on Saturday shortly after his WBC Latino Silver lightweight bout with Eduardo Javier Abreu in Argentina ended in a draw. He was taken to hospital and underwent emergency surgery but died on Thursday.
Santillan is the second boxer to die from injuries sustained in the ring this week after Dadashev’s death was confirmed on Tuesday.
The 28-year-old had been hospitalised with bleeding on the brain after his IBF light-welterweight fight against Subriel Matias was stopped at the end of the 11th round last Friday.
He underwent emergency surgery but failed to recover.
Boxing promoter Kalle Sauerland tweeted on Thursday: “A sad, sad week for boxing.”