The 29-year-old told the club's official website: "With players being away on international duty it's meant they are all coming back in dribs and drabs.
"It's been a little strange and as a result we've not had much time to prepare ahead of our matches. We're not performing as well as we should be but hopefully we will start turning the corner now.
"I do believe that when the Cliftonville game does come around our lads will be fully focused and the lights will go on for the big game. After that we will be bang up to speed."
Celtic reached the knockout rounds of last season's Champions League and defeated Barcelona in the group stages.