Ian Rapoport of the NFL Network reported Dissly suffered a torn patella, which is a season-ending injury.
Dissly, a fourth-round draft pick, got off to a strong start in his rookie year. He had seven receptions for 151 yards and two touchdowns in his first three games. He had one catch for five yards Sunday before exiting.
By losing Dissly, Seattle is now down to one healthy tight end: Nick Vannett.
Ed Dickson is already out for at least the first six games of the year after the Seahawks placed him on the reserve/non-football injury earlier this month.
Seattle will almost certainly have to sign another tight end—even as a short-term solution—ahead of its Week 5 matchup with the Los Angeles Rams.
The Rams rank second in pass defense DVOA (defense-adjusted value over average), per Football Outsiders. The Seahawks entered Week 4 ranked 22nd in passing offense (208.7 yards per game), so they can ill afford to not strengthen their tight end depth prior to a difficult divisional test Oct. 7.