The Algerian international was one of two players - along with Martin Dubravka - to head to Tyneside on loan during the final hours of transfer deadline day, after making just two starts for Leicester City this term.
But while Dubravka made the dugout for Saturday's clash against Crystal Palace, Slimani was absent from the match-day squad in his side's hard fought 1-1 draw at Selhurst Park.
However, the 29-year-old is likely to be deployed against United as Benitez will concede his side will have the minority of possession on the day and will need the ball to stick up top.
"He [Slimani] was not ready," said the Magpies boss, as quoted by the Chronicle Live.
"He was training on Saturday, he will train tomorrow, and he will have a chance for next Sunday. He must compete, like Dubravka and Kenedy."
Slamini netted five times in 17 appearances for the Foxes this season; averaging a goal once every 129 minutes.
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It's no secret that Newcastle are seriously lacking firepower in the attacking third this campaign - scoring just 23 goals in 25 league matches - and have been dragged into the relegation dogfight as a result.
The Toon faithful will hope the former Sporting CP striker will be the answer to their current goal-scoring woes. Netting against second-placed United would certainly be a good place to start.