Rotoworld is reporting a case of dead arm, which would explain the velocity drop. here's their post.
Brandon Webb has experienced decreased velocity recently and said Wednesday that he may be going through a "dead arm" period.
Webb was pulled from his start Wednesday after just five innings, with manager Bob Melvin saying afterward that he "just wanted to give him a little blow." Webb's fastball was clocked at 85-89 mph Wednesday and his next scheduled start will come Tuesday against the A's.
I'd expect the D-backs to take it easy on him next start as well, or even an extra day off if they can be lucky enough to get a rain out in the next few days. Regardless he's still a must start and it's not time to worry