Dumervil left John Fox's side in bizarre circumstances during free agency, as he failed to file his contract documents back to the club in time so they were forced to release him.
The 29-year-old then rebuffed another offer from the Broncos to join Super Bowl champions Baltimore Ravens.
"We certainly know what a great athlete Elvis is and what a playmaker he is," Manning told reporters. "So he's certainly a factor we have to deal with."
Denver face the Ravens in the NFL season opener on Thursday