The move triggers the final two years on Thomas' deal and guarantees him $12.5 million for 2018 -- his $8.5 million base salary and a $4 million bonus, the Denver Post reported, citing a source.
Thomas, 30, had 83 receptions for 983 yards and five touchdowns last season. It ended a string of five consecutive seasons in which he had at least 90 catches and went over 1,000 yards receiving.
Denver reportedly reached agreement on a two-year deal with Keenum, hoping it ends the team's carousel at quarterback.
Thomas signed a five-year, $70 million contract in 2015 but his production has declined despite the high volume of receptions.
The former Georgia Tech product has 16 touchdown receptions over the past three seasons after hauling in 35 over a three-year span from 2012-2014, including a career-best 14 in 2013.