The 29-year-old has travelled to his home country Algeria for extra treatment and is unlikely to be available until the Premier League home fixture against Southampton on March 10.
"Almost everybody is available apart from Jesus Gamez, and Slimani had a set-back the other day, so he will not be available," Benitez told reporters ahead of Newcastle's league trip to Bournemouth on Saturday.
"It will at least be a couple of weeks, we have to be careful with him because he had a problem when he arrived.
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"We worked really hard to try to bring players in to give us something different in the Premier League. We knew he was carrying a small injury, he'd had little problems but in the last day he had to stop and check again."
Benitez expects Slimani to feature for Newcastle sooner rather than later but is pleased with the quality of players at his disposal, having produced a 1-0 league victory over against Manchester United this month.
"Everyone was expecting him to make the difference, but after the game against Manchester United we at least feel we have enough quality in the squad," the Spanish manager added.
Newcastle are currently 13th in the league but remain just two points clear of the relegation zone with 11 matches remaining.
(Reporting by Hardik Vyas in Bengaluru; Editing by Keith Weir)
