The world's best player, who is coming to the Wolves on a short-term contract reputedly worth £10,000 a match, is due to make his debut against Leeds on Saturday.
He took a knock early on and limped through the rest of the game against St George-Illawarra. Johns kicked four goals as Newcastle lost 36-28.
"At half-time I was struggling to walk, but once I got out there and started running it was all right," Johns said. "I'll stay up all night and ice it, but it's just a bad bruise."
Johns has to be cleared by the Newcastle club doctor before he travels, but he remained confident of joining his temporary hosts on schedule.
"I'll have the medical, get some injections and see how many sleeping tablets I can eat on the plane on the way over," he said.
"It's simply a kick on the shin,'' said the Warrington coach, Paul Cullen yesterday. "I spoke to him this morning and he'll be on the plane tonight.
"He got the injury early and played 80 minutes of outstanding football, which shows the sort of player he is.''
Adrian Morley is free to take up his short-term contract with Bradford after his club, the Sydney Roosters, missed out on the NRL play-offs.
The Great Britain forward will make his first appearance for the Bulls in their match at home to Huddersfield on Sunday.Reuse content