I am not doubting Lewis' poor play and lack of confidence......but......
Last 3 seasons we've bounced between a back 3,4, and 5. In 3 seasons we've had Logan, Devlin, Taylor, Considine, McKenna, Leigh, Hoban, McCrorie, Hayes, Ramsay, McKenzie, and Bates, Gurr, and Gallagher. That's 14 players. 15 if you add Jon Gallagher. Plus we've had changes in management, coaches, and tactics/style of play. Have we ever really developed shape, formation, understanding, and continuity/chemistry? A strong, stable, and consistent defense always makes your keeper look better.
While I rate Ramsay and McKenzie, they are youthful and naive, and they don't have a solid and stable central paring to help them, or strong and consistent players around them. In the past we used to praise the work of McGinn and Hayes when helping Logan and Considine, or Shinnie, our young guys don't have the same experienced and effective support.
Lastly, have we had a consistent midfield protecting the defense, particularly a strong holding mid filling that central spot?? Again plays a factor in a shit defense and that affects' a dodgy keeper.