I personally don't think Lehtonen is a massive downgrade from Bishop, but something is definitely wrong in Dallas. They've had depth issues all year long and now the offense has dried up. I'm reading that fans are getting more than a little critical of Hitch and his staff and their lineup decisions.
As for Bishop getting hurt, again, this always seems to happen with him. Not his fault, but shit luck with injuries seems to follow him around. I think that, more than anything else, has led to him being a step below a legit franchise goalie.