He is 21, not 34. There's a better chance that he'll improve and get better next season. He's proved people wrong once, I wouldn't be so quick to say he won't be good enough to start next season.
In fact, I think Rodgers might even be confident or convinced enough to start him ahead of Johnson on the right at the start of the season, if we get a new left back. Especially since Johnson will rejoin training later, due to his involvement in the World Cup.