The arrival of the world's leading player at a sold-out Halliwell Jones Stadium co-incides with Leeds' first real slump under the coaching of Tony Smith, with their Challenge Cup final defeat coming between losses to Bradford and Wigan.
"They've got a great roster of players," Johns said. "They're coming off three losses, which makes them even more dangerous."
Initially, Johns will share responsibility at half-back with Lee Briers, with Nat Wood likely to start on the bench despite a man of the match performance against Salford last week.
Johns says he is looking forward to playing against Danny McGuire and Rob Burrow, two young British half-backs he rates highly. He is also likely to face Richard Mathers and Chris McKenna, who missed last week's game at Wigan with a stomach bug and an ankle injury respectively.
There are other debuts for short-term signings tomorrow, with the Great Britain forward Adrian Morley pulling on a Bradford shirt for the first time against Huddersfield and Sione Faumuina making his debut for Hull at Wakefield as the centre Kirk Yeaman is suspended.
Wakefield have signed the utility back Paul White on a two-year deal from Huddersfield, who have similarly recruited Andy Raleigh from Hull KR. Both will finish this season with their current clubs.
Finishing positions in the National Leagues will be decided tomorrow, with Whitehaven guaranteed first place and home advantage in the National League One play-offs if they avoid defeat at Rochdale. Should they slip up, Castleford could still pounce by beating Batley.