Benitez, already without Aleksandar Mitrovic, lost Dwight Gayle, Vurnon Anita and Isaac Hayden in Saturday’s 2-1 Championship win over Brentford.
United are also without Mohamed Diame, Christian Atsu and Chancel Mbemba, who are at the Africa Cup of Nations.
Sammy Ameobi and Jamie Sterry, back from half-season loan spells at Bolton Wanderers and Coventry City respectively, are cup-tied and unable to play in the third-round replay at St James’s Park on Wednesday night.
Jonjo Shelvey, however, is back from a five-game suspension, and the midfielder is likely to be involved against Birmingham, who held Newcastle to a 1-1 draw at St Andrew’s earlier this month.
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Asked if Shelvey would be held back for Saturday’s Championship game against Rotherham United, Benitez said: “We have to analyse all the options, because obviously we have lost some players.
“Ameobi and Sterry cannot play in the cup, so we need to analyse the options.
“He may have to play. Or I may have to play!”
Benitez had no choice but to take off Gayle, Hayden and Anita at Griffin Park.
“To make substitutions is always not good news, but in the end we had to do it,” he said.
“I’m really pleased with the players coming off the bench. They performed well in a difficult game.
“I am sure that the fans will be very proud of the team.”