Ince, on loan from Chelsea, was on the receiving end of a strong challenge and Johnson said he is unsure whether the midfielder will be fit for the start of the season.
"It caught him on the shin and when your leg goes numb you always think the worst so we don't know how bad it is," PA Sport quotes Johnson as saying.
"We keep our fingers crossed he is going to be fit and available for when the season comes around.
"Hopefully it's not a really bad injury. He's in a bit of pain and very upset that he has come all this way and missed most of the game."
Ince has yet to make an appearance for Chelsea's first team.