John Elway made a desperate move trading for Joe Flacco ... and it could end up being a HUGE mistake -- so says Elvis Dumervil, who tells TMZ Sports, "Flacco is no Peyton Manning."
Elway took a page out of his 2012 Broncos playbook Wednesday ... acquiring a veteran Super Bowl MVP with the hopes of revitalizing a tired Denver franchise.
The problem? Elvis says Flacco ain't even close to what Manning was when Denver picked him up 7 years ago ... and now the former Bronco is SERIOUSLY concerned for the team's GM.