In this scenario, Crawford gets forced out of the starting lineup, and presumably would be a backup at DE, because he is a little light at DT. I could see Crawford backing up 3 of the 4 positions on the DL.
I'm not for or against Crawford, but I think we need to see what we have in him. From a salary cap stand point it would be better to see Spencer be traded and Crawford start, but it is looking like the Cowboys don't have a lot of confidence in Crawford's abilities.
The problem with some coaching staffs is that they look at how a player will be playing on opening day, but do not consider how that same player would improve over the course of the season if he were to receive starting minutes. Spencer is likely to be the same player (barring injury) on week 1 as he is in week 17, or the playoffs. Whereas a younger player may improve dramatically over the course of the season, if he gets significant playing time.