The Champions League holders were beaten at Estadio Metropolitano as they found no way through a firm Atleti defence after Saul Niguez gave the hosts the lead just four minutes into the first leg.
Robertson rued the difficulties the Reds ran into as their La Liga opponents diligently sat deep and blocked space, but insists the tie remains very much alive.
“Tough one,” he told Liverpoolfc.com.
“Obviously we couldn’t have got off to a much worse start, especially for a team that likes defending leads and is really good at it. It was a tough one.