Johnson, 21, returned to Leeds last summer after taking a year out from rugby league, but missed their opening two pre-season games because of a back/leg problem.
Along with Muizz Mustapha and Jarrod O’Connor, he is one of a trio of Rhinos players who will be available to Bulls this year when not part of Agar’s matchday squad.
All three had an eventful afternoon in Leeds’s 30-12 win at Odsal yesterday.
Mustapha was sin-binned for fighting in the first half, along with Bradford’s Matty Dawson-Jones, as the hosts opened a 12-0 interval lead.
O’Connor set up Leeds’s first try straight after the break and Johnson called the shots in a full 80-minute effort.
Johnson has England squad member Kruise Leeming and Leeds’s 2021 top try scorer Brad Dwyer ahead of him in the pecking order, but Agar insisted he is a “genuine contender” to play in the first team.
The coach described Johnsonm’s performance yesterday as “outstanding” and noted: “He plays hooker, but he is really comfortable playing in the halves. He can play middle if you need him to and he’s the type of kid you could throw in many positions and he’s that smart he can carry it off.
“He has really shown [yesterday] what a capable rugby league player he is.”
Rhinos full-back Jack Walker, who has been plagued by a foot injury, came through unscathed in his first game since November 2020 and Liam Sutcliffe made a successful return following knee surgery.