Mike Klis of 9News reported Tuesday that the roster move is expected by the end of the week.
The Broncos acquired Flacco in a February 2019 trade with the Baltimore Ravens.
He struggled mightily during his first year in Denver, completing 65.3 percent of his throws for 1,822 yards with six touchdowns and five interceptions across eight starts.
Flacco didn't establish himself as a franchise quarterback after Baltimore took him in the first round of the 2008 draft. His 84.1 career passer rating ranks 24th among active QBs, per Pro Football Reference.
His shining moment was the 2012 playoffs when he suddenly transformed into the world's best quarterback for a month. He threw for 1,140 yards with 11 touchdowns and no interceptions in four games to lead the Ravens to the Super Bowl XLVII title.
He's otherwise never been able to match that elite level of production.
A trade or release would leave $13.6 million in dead salary-cap space for the Broncos, but it'd still represent $10.05 million in cap savings for 2020, per Spotrac.
Denver will move forward with Lock, who showed promise while posting an 89.7 passer rating as a rookie, as its starter for the foreseeable future.
At best, Flacco may land with a team where he can compete for the starting job during training camp, but he's nothing more than a short-term stopgap option at this stage of his career.