The Bhoys have already played 13 times competitively this campaign and will need to start resting players in order to give them the best opportunity of performing well in the Scottish Premier League, Champions League and cup competitions.
"Adam was tight in the hamstring and took a sore one on the ankle and we couldn't push him any more," he said.
"He was really tired, as a few of them are now. Everyone thinks we have a big squad - we don't, and we have had a lot of heavy games already.
"We are trying to balance it but it is not easy. If the game had been on Sunday it would have given us a lot more time to prepare properly.
"What we have to be aware of is injuries. With only a two-day recovery from a Champions League game you have a 40 to 50 per cent better chance of picking up more injuries than with a three-day recovery so we are looking at that as well.
"I will use my squad (against Morton) but it is a competition I want to progress in.
"I would like to keep the momentum going. The players are playing well but I have to strike a balance in terms of resting players and giving other players a chance to play."