Rivera fills the spot of the move tight end, which is the role Thomas had with the Jaguars the past two seasons. Marcedes Lewis is entering his 12th season with the Jaguars and is now more of a blocking tight end. He hasn't caught more than 25 passes in a season since 2012.
Rivera, 26, was the second of three sixth-round draft picks by the Raiders in 2013, after running back Latavius Murray but before defensive tackle Stacy McGee. He flashed at times in Oakland, despite being slightly undersized for a tight end at 6-foot-3, 245 pounds.
He caught four touchdown passes in each of his first two seasons, peaking in 2014 with 58 catches for 534 yards in 16 games (10 starts), but has been caught in a numbers game since, with the arrivals of Lee Smith, Clive Walford and Gabe Holmes.
Still, Rivera, who had a base salary of $1.671 million last year, had the trust of quarterback Derek Carr deeper down the field last season as his 10.7 yards per catch average equaled a career high. He also showed he could be a safety valve for rookie quarterback Connor Cook in the Raiders' playoff loss at Houston with four catches for 31 yards.
Last season he played in a career-low 13 games, starting a career-low two, and his 18 catches for 192 yards were also career lows, while his one touchdown equaled a career worst. He was purportedly on the trading block in training camp.
Rivera, whose sister is actress Naya Rivera from "Glee," averaged 1.10 yards receiving per route run, which was 65th out of 133 tight ends in the league, per Pro Football Focus.
ESPN's Paul Gutierrez contributed to this report.