Dummett and Fernandez sat out Saturday’s 2-0 home defeat to Leicester City with hamstring problems.
Benitez was forced to play Kenedy at left-back in the absence of Dummett, while Ciaran Clark was recalled in place of Fernandez.
Striker Salomon Rondon also missed the Leicester game with the thigh injury he suffered in the warm-up at Selhurst Park the previous weekend.
Asked about Dummett and Fernandez, United manager Benitez said: “They’re light injuries, so not long. They’re not very bad.
“We had to make a decision, and in the last two games, we had a problem with Rondon and Lascelles and we had to make a substitution during the game.
“We preferred to go with players who were fully fit. That’s it.”
Rondon will miss the Manchester United game.
“We have to assess him in the next weeks and see how he is going on,” said Benitez.
“The other players are training and we will have to make a decision later.
“I think a couple of weeks, maybe. The idea is to see how he is going on after every week.
“He is injured and now we need to manage with the rest of the players.”
Rondon joined Newcastle on a season-long loan from West Bromwich Albion in August.
Meanwhile, Florian Lejeune is due back on Tyneside this month to step up his rehabilitation.
The Newcastle defender ruptured his anterior cruciate ligament in pre-season.
Lejuene has been working in Italy since undergoing an operation in Rome early last month.
The 27-year-old – who has been posting updates on Instagram – could be back early next year.
Asked for an update on Lejeune’s return, United manager Rafa Benitez said “He is getting better – you have his Instagram!
“He will come back maybe in one week He’s doing his rehab in Rome, special facilities.”