Rodgers, who has criticised several aspects of the Hoops' backline in recent games, was left to rue another example of lacklustre defending that left his side with a difficult job in the return leg.
"We gave away a soft goal," the Northern Irishman told reporters. "That is a real concern. We are not giving away many chances, but we don't defend the set-piece well enough.
"It is a European goal away from home and we go back next week to finish the job, but I look at the performance level and it wasn't quite how we wanted.
"We seem to lack concentration. That is a problem for us and our pride in terms of defending. It was a terrific ball into the box, but we have to defend it better than that. It gave them a bit of oxygen in the game that really our start should never have given them."
Celtic will contest the second leg at Parkhead next Thursday.