"Home games are very important in the Champions League," he said. "If we win this one, we will take a huge step forward.
"Celtic drew at home to Benfica and beat Spartak [Moscow] and that's a sign they are doing things well. Maybe people didn't take Celtic seriously at the start of the season, but we do.
"Over the years there have been lots of examples of teams who have been written off and were dangerous. Celtic are very quick and good on the counterattack, so we have to take them seriously in the Champions League."
Celtic are second in the group at the moment with four points from two matches, two points behind leaders Barcelona.