The 21-year-old striker built on his penalty opener by sliding home his side's second from close range 10 minutes later.
Freedman said: "Marvin has done very well for us. He is only a young guy and there is a lot of pressure on him with a big move up to the north then being called into the Olympic squad.
"With my experience of working with young players like Marvin, we got him back to basics and stripped down his game and he's started to prove what he is - a goalscorer."
Freedman said his side's game-plan had paid off as they soaked up early Sunderland pressure before pouncing in the second period to seal a fourth-round home clash with Everton.
Freedman added: "We had a game-plan and I felt we stuck to it. I think we defended very well and made sure in the first 60 minutes we limited Sunderland to a few opportunities.
"We grew into the game with our passing and ability to control the ball and over the 90 minutes I felt we deserved to win the match.
"We're looking forward to it (Everton). We feel right now that we can perform against anyone and if we defend like we did and keep our discipline, we have definitely got every chance."