Porter suffered the injuries during a closed scrimmage against Southern Illinois.
He is the younger brother of Michael Porter Jr., the former top recruit who missed the majority of his lone season at Missouri last year before being drafted in the first round of the NBA draft by the Denver Nuggets in June.
Jontay Porter, a 6-foot-11 power forward, averaged 9.9 points, 6.8 rebounds and 1.7 blocks last season and was expected to see a jump in production in 2018-19. He was named preseason second-team all-SEC last week.
Porter tested the NBA draft waters last spring but decided to return to the Tigers for his sophomore season, despite being projected as a late-first-round pick in multiple mock drafts.
ESPN's Jonathan Givony pegged Porter as the No. 13 pick in his 2019 mock draft in August.
Thanks to everyone who reached out! It's hard to stay positive in the midst of something like this but everything happens for a reason.— Jontay Porter (@JontayPorter) October 22, 2018
Jontay Porter graduated high school a year early to play with Michael, but his brother played just three games last season and had back surgery in November.
Knee problems have been an issue for the Porter siblings in recent years. Bri Porter, Jontay and Michael's sister, retired from basketball before last season at Missouri after suffering her fifth ACL tear. Cierra, another sister who played at Missouri, medically retired in June -- with one year of eligibility left -- because of ongoing knee problems.
Last month, Missouri announced that guard Cullen VanLeer would retire after tearing the ACL in his right knee in the Tigers' final regular-season game last season.
When a reporter told Kentucky coach John Calipari about Porter's injury after his team's intrasquad Blue-White game on Sunday, he looked down and shook his head.
Calipari then told a story about Nerlens Noel, the former Kentucky standout who tore his ACL during the 2012-2013 season. He said Noel began to cry uncontrollably when Calipari hugged him on the team plane after he suffered the injury in a game against Florida.
After Noel's injury, the Wildcats stumbled and lost in the NIT that season.
"I've been there," Calipari said of Porter's injury. "Thoughts and prayers go to him and his family. That's awful."
ESPN's Myron Medcalf contributed to this report.