The midfielder conceded: "I don't think any of us were good enough tonight. We could've done a bit better at home, individually and collectively."
"Credit to Barcelona I thought they were outstanding."
Barcelona assistant coach Johan Neeskens acknowledged that it was a difficult place to come, but believed his side merited the victory.
"We know it is very difficult to play here at Celtic Park. Over the whole 90 mins, we controlled the game and deserved to win."
"We have to take the next leg very seriously and we will have to give 100% at home to get a good result," added Neeskens.