Craig Brown, thanks for your help!

"haven't simply gone out and hurled a diamond the size of the moon at his head" = paid him a lot of money just to get him on the team. This is not a common expression. The writer is using a metaphor. In this paragraph, it means that "City" didn't just give Messi a lot of money to get him to sign on to the team. They are also putting effort into getting Messi to play at his best.