Scans and more scans would surely show if his foot was in the danger zone or not. Perhaps they just weren't anticipating something like that happening. What you mention about Eduardo and Ramsey is slightly different - horrible injuries though they were (and in Eduardo's case much worse) broken bones take longer to heal (obviously) but eventually bones fuse back together stronger. In any case I don't really want to get bogged down in a compare and contrast with those players because that doesn't really preclude from the possibility of being a mismanagement in Wilshere's case.
Having said that, they are having more tests for him. Granted it could be after the fact but then, it might not be as bad as expected.