The deadline-day loan signing from Leicester City wasn't involved in Newcastle United's 1-0 win over Jose Mourinho's side at St James' Park.
The Algerian forward arrived on Tyneside with a thigh injury and Newcastle, who have a two-week break ahead of the January 24 game against Bournemouth at the Vitality Stadium, can now get him fully ready for his debut.
Manager Benitez said: "He was fine, but he was still feeling it a little bit.
"It was a case of waiting because we have 15 days now before the next game so we can keep working with him."
Slimani, who was unveiled on the eve of the Manchester United game, admitted he would be 'very happy' to win a permanent deal at Newcastle.
Asked if he was open to staying at the club beyond this season, Slimani said: "Yes, yes. I'm just on loan at the minute, but if I could stay here I'd be very happy, so let's see how it goes."