The 29-year-old Leicester striker, who joined the Magpies on loan on deadline day last month, arrived nursing a thigh problem which he aggravated in training this week in his efforts to make himself available for Saturday's Premier League trip to Bournemouth.
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Slimani, whose last league appearance for the Foxes came on New Year's Day, will be sidelined for at least two weeks.
Newcastle manager Rafael Benitez said: "Slimani had a set-back the other day, so he will not be available. It will at least be a couple of weeks. We have to be careful with him because he had a problem when he arrived.
"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.
"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. Hopefully he will still help us in the next games."
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The Magpies turned to Slimani after their efforts to land Liverpool's Daniel Sturridge ended in failure - he too is injured after joining West Brom - but with the Leicester frontman unable to play against his parent club, he is now likely to be available for a maximum of seven games.
Benitez said: "When you go to the last day (of the window), it is always a risk. We did it and now we have to manage in the best way possible.
"He trained really hard. I was talking with him and he was positive. When he was feeling well, he was training really well, but then he felt this little problem."
Slimani's misfortune comes at a time when there is real positivity within the Newcastle camp following the shock 1-0 win over Manchester United in their last outing.
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