Utah coach Kyle Whittingham said Bowen, who transferred from Penn State, left the team earlier this week to pursue a business opportunity.
"Unfortunately, a couple days ago, Manny informed us that he is retiring from football," Whittingham told reporters in Salt Lake City. "He's got a business opportunity that he felt was too good to pass up, and the opportunity was for right now. He is exiting football and starting a business career. We wish him the best, hate to see him go. We were excited about what he was going to bring to the table this season."
Bowen couldn't be reached for comment Thursday.
"It surprised us completely, because there was no hint or anything out of the ordinary all through summer," Whittingham said. "All of the sudden, it just came up."
Bowen was a projected starting linebacker for the Utes after joining the team for spring practice. He announced he was transferring to Utah in November and graduated from Penn State the next month.
Penn State dismissed Bowen from the team following its bowl game after the 2017 season for what the school said was a violation of unspecified team rules. After briefly returning during training camp, he left the team before the 2018 season began. In three seasons at Penn State, Bowen made 21 starts and appeared in 35 games with 134 tackles and 12.5 tackles for loss.
The Utes must replace their top two linebackers from 2018 -- Cody Barton (116 tackles with 10.5 tackles for loss) and Chase Hansen (114 tackles with 22 tackles for loss).
The Utes, ranked 15th in the initial coaches' poll that was released Thursday, open the season at BYU on Aug. 29 (10:15 p.m. ET, ESPN).