Portland State Vikings Linebacker AJ Schlatter, 20, died Sunday following complications resulting from minor throat surgery. The University announced the news on Monday.

Portland State Linebacker AJ Schlatter Dies After Tonsillectomy

Schlatter, the Vikings’ fourth-leading tackler, helped his team upset the Washington State Cougars and reach the Football Championship Subdivision playoffs this past season, his second one in the program.

“My heart is with the Schlatter family. They are our focus right now,” Portland State head coach Bruce Barnum said in a statement. “We lost a diamond that will never be replaced. He was the definition of what we are about at PSU.”

Schlatter’s father, Jim, posted additional details about his son’s death on Facebook, and revealed that he believes his son may have developed a blood clot after his tonsillectomy on Friday. “He was such an inspiration to me and I was and am so PROUD to be his father,” Jim Schlatter added in his post. “In his 20+ years he became a greater man then I could ever dream of becoming!”

During his last season with Portland State, Schlatter played 11 games and started 10, racking up 62 tackles (6.5 for loss), two fumble recoveries, two sacks, and an interception. His death follows a few other notable sports deaths in the first three weeks of 2016, including former Butler University Basketball player Andrew Smith, who died of non-Hodgkin’s lymphoma on Jan. 12.