The pair entered the fray in the space of a few minutes either side of the hour mark with the Hammers looking dangerous in a tense third round encounter in front of a packed house at the Lane.
Helder Postiga's arrival soon after, with Robbie Keane dropping into midfield, completed the turn around.
In the end, Bobby Zamora bagged his first goal for the club just two minutes into extra time to seal the 1-0 win and a home tie against Manchester City in round four.
"With respect, I think our changes altered the shape of the game," said caretaker boss David.
"Mabizela and Blondel were important for us, Ledley King went into a deep-lying midfield role and Robbie Keane did well as the head of the diamond in midfield.
"Paul Konchesky had to go to full-back because of Christian Ziege's injury and I felt the substitutes helped us.
"I thought that gave West Ham problems. I was pleased with the two boys."
David Pleat highlighted the contribution of young substitutes Jon Blondel and Mbulelo Mabizela after last night's Carling Cup win against West Ham.