Diego Costa has withdrawn from Spain's current squad due to a hamstring problem. The Spanish Football Federation had the following to say about the player and his injury:
"Diego Costa is not available for selection because of an injury to the hamstring muscle in his left thigh. All imaging tests will be sent to Chelsea."
Costa was questionable for Chelsea's thriller against Everton with a similar hamstring problem, but started and played very well in the 6-3 victory. If the problem was remotely serious, it didn't show at all based on his two-goal display.
With that in mind, I'd be surprised if this was anything but the player being cautious with his body during a relatively meaningless international break, as Spain are at very little risk of failing to qualify for Euro 2016. I'm fine with that approach, and wish we saw it more often. Rest up Diego, I'm just starting to get used to having a quality center forward again, and would hate to see you on the shelf for any length of time.