There's a difference between a "little gamble" and taking on a complete novice.
That rather depends on the alternatives. We could employ plenty of managers that could get us regularly in the top ten but we can't buy our way any higher so have to try and weed out those with potential. If Arteta is highly thought of by those he has worked with and has a growing reputation he will be a gamble worth taking.
We've got it all to gain and little to lose.