Regular quarterback Christian Ponder came in for criticism during the 31-27 loss to the Cleveland Browns last weekend, with some sections of the crowd calling for Cassel to replace Ponder midway through the game.
With Ponder now struggling with a rib injury, the Vikings may have no option but to use Cassel against the Pittsburgh Steelers at Wembley Stadium in their upcoming match.
Brady - who used to have Cassel and Cleveland Browns quarterback Brian Hoyer as back-ups - told reporters on Wednesday: "I'm friends with both those guys. Matt Cassel, who may be starting this weekend. And Brian, I text back and forth."
Cassel was with the Kansas City Chiefs last season and is yet to play a regular season match for the Vikings.