Brendan Rodgers is wary of the threat Manchester City's Sergio Aguero will pose for Celtic in the Champions League on Wednesday night.
Despite missing three Premier League games through suspension, Aguero has already scored 11 goals in all competitions so far this season.
Celtic boss Rodgers faced the Argentine in his time in charge of both Swansea City and Liverpool, and knows what to expect from the forward, even if he is not City's only danger man.
"It is a difficult game of course," he said. "I have come across Aguero many times, I know his qualities and know the level he is at.
"His first goals in English football were against my team Swansea when he came on as substitute - and he actually cost more than our stadium at Swansea cost.
"He is a big talent, one of the leading strikers in the world, but as a team they are operating at a really high level. They are world-class players working really hard."
Scottish champions Celtic, who beat Kilmarnock 6-1 on Saturday after falling a goal behind, were thrashed 7-0 by Barcelona in their Group C opener.