Blackwell, who refused to reveal the identity of the injured duo, will be without on-loan Manchester United winger Lee Martin, who has a knee ligament injury, and the suspended Stephen Quinn.
Quinn will sit out the first of a three-game ban following his last-minute dismissal against Charlton.
Blackwell revealed he was ready to make changes at Bramall Lane after Saturday's 2-0 defeat and could be tempted to switch his strikers.
James Beattie and Billy Sharp could make way for second-half replacements Rob Hulse and Jon Stead in the starting line-up, while David Carney and Keith Gillespie will both be hoping to return to the starting line-up.
In Ipswich's camp, Former Blade Alan Quinn is doubtful with an ankle injury so Owen Garvan is set to replace him.