The Canadian full-back, who recently returned from an August hamstring injury, was forced off the pitch around 20 minutes after coming on.

O'Neill told Sky Sports: "Alistair's felt something again a little bit high up in his hamstring and we were a wee bit concerned about it."

He further elaborated on the uncertainty surrounding the player's condition: "We don't know, we'll have to see.

🗣️ “Given the conditions, given the fact that we didn’t play all that well, I thought it was our best win of the season.” 🤕 Alistair Johnston “felt something high up” Martin O’Neill on Celtic’s 2-1 victory at Easter Road 🍀 https://t.co/q5stV1ZJZN

— Sky Sports Scotland (@ScotlandSky) October 25, 2026

"That would be a blow if he's out.

"You never know, it might be cramp, might be a number of things but he was really settling in brilliantly in the space of time he was on the field."

When asked if this indicates a potential injury relapse, O'Neill was cautious: "I don't know, I don't want to think about something that may not be there."

He revealed a conversation with Johnston at full-time, adding: "I've just spoke to him in the dressing room and naturally when you're coming back from these particular injuries, you do have to have concern about it.

"We'll see how it is but we just don't know at the minute."