Georgios Samaras has claimed that nerves played a factor in Celtic's Champions League defeat to Benfica on Tuesday evening.
The 2-1 loss in Lisbon prevented the Scottish champions from securing qualification with one group game remaining.
Samaras said: "When you lose a game you are never happy. We did not play our best, we lost the game on the details, but the effort was there.
"Now we have to look forward and try and beat Spartak. We were nervous throughout, I don't think we had clear minds, but we have to concentrate on the group energy that we had."
Celtic face Spartak Moscow in their final group match and they will need to better Benfica's result against Barcelona if they are to qualify for the last 16.
