PORTLAND, Ore. (AP) — Portland State is mourning the sudden death of a freshman linebacker.

AJ Schlatter died Sunday night, the school said. James Schlatter said in a Facebook tribute to his 20-year-old son that the death came after a tonsillectomy Friday.

Schlatter, a redshirt freshman, was a walk-on who became a starter and helped Portland State make the FCS playoffs. The Vikings were ranked No. 10 in the final FCS poll, finishing 9-3 in a six-game improvement from 2014.

"We lost a diamond that will never be replaced," coach Bruce Barnum said in a statement Monday. "He was the definition of what we are about at PSU."

Schlatter finished fourth on the team with 62 tackles. He was honored as the Big Sky's Defensive Player of the Week for his play in a 34-31 victory over Eastern Washington late in the season. He had 12 tackles, two sacks a forced fumble and an interception.

"He was such an inspiration to me and I was and am so PROUD to be his father. In his 20+ years he became a greater man then I could ever dream of becoming!" James Schlatter, an assistant coach for the Vikings during the 1980s, posted to Facebook.

AJ Schlatter's mother, Terri Jo, played volleyball for the Vikings and was also an assistant coach of the team in the 1990s.

Schlatter was a standout at Canby (Oregon) High School before joining Portland State. He redshirted his first year and earned a scholarship last fall.

Former coach Nigel Burton also lent his condolences to the Schlatter family.

"AJ was like a son and brother to so many in the Portland State football family," Burton wrote in a post to Instagram. "I hurt especially for Mr. Schlatter. You didn't fail. You succeeded more than you know. You raised a son that so many people admired and whose hearts AJ touched in such a short time on this Earth. We should all be so lucky."

Portland State said funeral arrangements were pending.