Robertson has warned his team-mates to improve or risk further damage in their Community Shield clash with rivals and Premier League champions Manchester City on Sunday.
He said: “It wasn't good enough. We were miles off it. We were outplayed. At times we looked foolish by not pressing as a team.
“And we need to look at it and improve as our season starts in a week and it doesn't get any harder than Man City.
“We need to be a hell of a lot better than this.
“But we can't get into a habit of losing or drawing. We need to step it up massively."