The 20-year-old Schlatter's father Jim wrote a post on social media Monday saying he believed his son developed a blood clot after having his tonsils removed Friday.
"He was such a inspiration to me and I was and am so PROUD to be his father," Jim Schlatter wrote.
Schlatter, a native of Canby, Oregon, was a redshirt freshman and starter for a Vikings squad that finished the season ranked No. 10 in the nation after making the FCS playoffs for the second time.
"My heart is with the Schlatter family. They are our focus right now," head coach Bruce Barnum said. "We lost a diamond that will never be replaced. He was the definition of what we are about at PSU."
Originally a walk-on, Schlatter earned a full scholarship with the program through hard work. As a freshman, he had 62 tackles and was named the Big Sky's Defensive Player of the Week following a win over Eastern Washington on Nov. 21.
Schlatter came from a family that's been ingrained in Portland State athletics. His mother Terri Jo played on the school's national championship volleyball team in 1984, became an assistant coach with the program and is a member of the school's athletics hall of fame. His father spent time in the 1980s as an assistant football coach, while one of his sisters, Garyn, was a star volleyball player for the Vikings.
"Our thoughts and prayers go out to the Schlatter family with all the compassion and sympathy we can bring to bear during this difficult time," athletic director Mark Rountree said.
"Things like this aren't supposed to happen. We cherish every student's well-being, and the loss of AJ will be tough on our family. He represented his family and Portland State with passion, courage, and humility that made us all proud. He will be missed."
Funeral arrangements were pending.