The 29-year-old discovered earlier this week that he had failed to make Martin Johnson' 30-man selection for September's tournament in New Zealand.
"I'm now resigned to the fact that my opportunity has gone," said Simpson-Daniel.
"I'll put it to the back of my mind and go all guns blazing for Gloucester.
"But I can't flick a switch, I'm obviously very disappointed. It's my last opportunity to play in a World Cup. It's tough but fortunately I've been there before so I'll just get my head down."
The winger has spent his entire career at the Kingsholm Stadium with Gloucester.