Neil Lennon has joined Aaron Hughes on Northern Ireland's casualty list ahead of Wednesday's World Cup qualifier with Bulgaria.
The Celtic midfielder hurt his ankle on the hard, bumpy surface during Tuesday's session at CSKA Sofia's training ground.
And he could be doubtful for the Group Three game which Northern Ireland badly need to win.
"I've just twisted my ankle and hopefully I will be okay," said Lennon.
Northern Ireland are already sweating on the fitness of Hughes after the Newcastle defender sat out the entire session. He received more treatment from physio Terry Hayes, sitting with his leg up and an ice pack on his calf muscle.
Like Lennon, Hughes remains optimistic he will recover in time to face Bulgaria and said: "I just got a knock in training yesterday and hopefully I will be all right.
"It's not a pull or a strain or anything like that, so it's not too serious."
Jim Magilton is fit to lead the Northern Irish and he trained after shrugging off his hamstring injury.
Michael Hughes missed the session but that was just a precaution because of the uneven surface and the fact that he has just returned from a broken leg.