The 21-year-old has struggled to break into the Millers’ first team since his move from Brentford in August, making just two appearances in the Checkatrade Trophy.
Onariase, who watched Saturday’s win at Barnet, spent the second half of last season on loan at Whaddon Road and will again hook up with the Robins, though will be unable to play until January 6.
“I don’t think he has come here if he isn’t joining us,” Johnson said.
“It’s 99.99 per cent certain that he will be joining us and he will link up with us on Boxing Day, although he can’t play yet.
“It’s good that we have got him in before he can actually be selected on January 6.
“He is looking forward to it and it’s great that he came here and that he wanted to see what he is coming into.
“He improved with us before and he is a lovely lad. We look forward to seeing the best of him.”