Let's get to the links...
The 10 players the Eagles can least afford to lose to injury - PhillyVoice
If the Eagles lost Johnson, they would be in trouble in the short term for similar reasons as the ones noted above with Peters. However, the difference here is that the Eagles will eventually need Johnson to kick over to left tackle whenever Peters' inevitable decline occurs. I do not believe the Eagles are serious Super Bowl contenders in 2015. Therefore, I think the short term loss of Peters wouldn't be as meaningful as the long-term effects of Johnson's stunted growth.