Oscar's goal in the Czech Republic last week puts Chelsea in the driving seat for the home leg, but Cahill insists there will not be any complacency at Stamford Bridge.
"You've always got to take teams seriously," he said. "They're a quality outfit, and made it tough for us away from home.
"But we got a positive result, the away goal, but we still won't take it lightly. They've got some great players.
"Our home form in Europe is nine unbeaten, so hopefully that can continue."