However, Rafa Benitez believes that the on-loan striker could still make a significant impact on the field.
Slimani, signed on loan from Leicester City, arrived on Tyneside with a thigh problem.
The 29-year-old suffered a setback in training last week and the earliest he could feature would be against Tottenham Hotspur at Wembley on March 16, though that game will be postponed if Mauricio Pochettino's side beat Rochdale in a fifth-round FA Cup replay this week.
Slimani might have to wait until the March 31 home game against Huddersfield Town to make his debut.
Benitez spoke about Slimani after Saturday's 2-2 draw against Bournemouth at the Vitality Stadium.
Asked what kind of difference Slimani could make to his team, Benitez said: “We have to see if he's fit, first.
"If he's fit, we will see. But Dwight (Gayle), Ayoze (Perez) and Joselu, if they continue working like they worked in the week, and then afterwards they score goals, that will be very good.
"He (Slimani) will have to compete for his position. Hopefully, he will be another body and before the end of the season, he will be another one who can help.
“The point is to win these kind of games, because maybe then you can manage the other games much more easily.
"Maybe we will have to keep going to the last game, but whatever happens, we will have to stay calm and make the right decisions every time.”
Dwight Gayle scored both Newcastle goals against Bournemouth to take his Premier League goal tally for the season to five.
“Dwight was complaining about a little bit of tightness in his hamstrings, so we needed to make a substitution," said Benitez.
United are 15th in the Premier League, and two points above the relegation zone, ahead of Saturday evening's game against Liverpool at Anfield.