The 29-year-old, who arrived on loan from top-flight rivals Leicester City on deadline day last week, has yet to feature for the Magpies due to a thigh complaint.
Despite paying a hefty fee to take Slimani for the rest of the season, rumoured to be worth around £2m in total, Benitez insists that he will not rush the Algeria international back to fitness.
"We have to see about him, we will see how he feels tomorrow and how fit he is," he told reporters ahead of Sunday afternoon's home match against the Red Devils. "I am not desperate, I am desperate to see all the players fit to give me problems in terms of options.
"He was part of the training session on Thursday and part of a session today in the gym, so we will see tomorrow what he is like."