The Serbian international spend half of last season on loan with the 'Bluebirds', finding the net once in 14 appearances.
Grujic impressed in South Wales and Warnock has made no secret of his desire to bring the 22-year-old back to the club this summer be it on a permanent deal or another loan.
Speaking following the 1-1 pre-season friendly draw with Torquay on Friday, the veteran boss told 'Sky Sports' that he remains hopeful a deal can be struck.
"I'm always optimistic. You've got to be at my age," he joked.
"We were a bit thin on the ground (in midfield) against Torquay in that area without Aaron Gunnarson as well so it shows we've got to make sure we get somebody in, in that position."
Cardiff have already enjoyed a reasonably successful summer, bringing in Bobby Reid, Greg Cunningham, Josh Murphy and Alex Smithies, but Warnock says no new faces are close to being added to the squad.
"No not really," he explained.
"A couple of new names have been mentioned. There are always going to be names crop up that will probably surprise you and you think 'wow, didn't think he'd be available' but sometimes they are."