We have 'a gazillion' injuries every single year. It can't be luck if it's always happening so you have to take the injury prone players into account when you're building a squad. Players like Diaby, Rosicky, Wilshere and Gibbs should be expected to miss long periods of the season.
I think Wenger did gamble to an extent. He offloaded a lot of deadwood, which was a good thing, but only brought in two players for our first team squad. It was a gamble to go into the season with only Bendtner as Giroud's backup. He had time to get someone else even if it was only a stop gap until January.