The deadline day signing wasn't involved in Newcastle United's 1-0 win over Jose Mourinho's side.
Slimani, signed on loan from Leicester City, had hoped to feature against Man United at St James's Park.
However, the striker – who arrived on Tyneside with a thigh injury – didn't make Benitez's squad.
Newcastle now have a two-week break ahead of the January 24 game against Bournemouth at the Vitality Stadium.
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 was unveiled on the eve of the Man United game.
The Algeria international said 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.”