Southampton defender Jake Vokins ready for Sunderland debut
Johnson does not think the injury is serious but the defender will be assessed ahead of the return trip on Saturday.
"Callum just felt his groin tighten," Johnson said.
"You can see the pitch is difficult. It’s quite firm just under that top inch of the surface, but the top inch is quite gunky and he just slipped as he sent the ball down the line.
"I don’t think it’s too bad, it’s just a bit tight, so we thought the best thing to do was to change it."
Vokins was on the bench for the 2-2 draw with Gillingham on Saturday and Johnson is confident that he is ready to step in when needed after making a positive early impression on the training ground.
"Jake is fit and ready to go," Johnson said.
"He couldn’t play tonight because he is cup-tied.
"With last Saturday, he'd obviously only been in for half a day. But he is ready to go and he has impressed us in training."
Fellow January signing Ross Stewart will be absent on Saturday, with Johnson hopeful that the striker will join full training on Monday after recovering from a hamstring problem.