United host the Bluebirds (kick-off 3pm GMT) as they go in search of a victory which would lift the Magpies out of the relegation zone after slipping into the bottom three following last weekend's 2-1 defeat at Chelsea.
Shelvey and Dummett - who both missed the trip to Stamford Bridge - will also sit out the match at St. James' Park as they continue to recover from thigh and hamstring injuries respectively.
After Ciaran Clark and Jamaal Lascelles were taken off in Tuesday's FA Cup win at Blackburn, as well as Fabiän Schar and Issac Hayden suffering knocks, the Magpies manager assured supporters that those injury concerns were not as alarming as many feared.
Benítez said: "We have to assess some players (after the FA Cup game) but I am quite optimistic with Hayden, Schär, Clark and Lascelles - they can be available if everything is fine.
"Jonjo Shelvey and Paul Dummett are still out. They are getting better but they are still not ready. Mo Diamé was training today but we still have to assess him."
With both sides in desperate need of three points to move further towards Premier League safety, Benítez reflected on the terrific support shown in the third round replay as well as sending out an appeal for the strong backing of his side to continue for Saturday's crunch clash.
"We know where we are and we know we have to stick together and keep working hard," the Spaniard added. "If you want to take positives from this week - we've won a tough game with the fans wanting us to play with a strong team and to win.
"We were managing the squad and we still won along with scoring four goals. There were around 2,000 away fans which shows how good they really are, supporting the team and always being behind us, which is a key factor for the future.
"We play against Cardiff now. From the beginning, they will try to stop our team doing what we want to do to make our fans nervous and anxious. The message is clear - stay together behind the team and keep pushing to help the players because it's our team. If we want to stay up, we need every single fan behind the players whether we are doing well or not.
"It's an important game, but we still have a lot of games to play. We will go for the three points and after that, we still have to keep winning games. If we are together and we continue pushing and working hard, we will have more chances for sure."
