According to Carwin's website:

"I am going to pull out of the fight with Roy Nelson. As many of you know I have been having some back pain. I had an MRI yesterday and I have some damage that may really require surgery. If the doctors do not have to perform surgery then I will be out 8-12 weeks. If they do have to perform surgery I do not know how long I will be out of action."

"I was really looking forward to fighting again and obviously this is a very disappointing turn of events. My manager said "You can’t fight the best fighters in the world with just heart, it is time to stop limping to the top of the mountain and get healthy so you can climb it on your terms." I have been up all night thinking about this decision and what I may be missing by making it."

Roy (Nelson) and my (Engineering) boss are the only ones happy about this

"It would be an interesting fight for sure. I think Roy (Nelson)’s style matches very well with mine, we could have a good fight. I didn’t hear anything from the UFC yet, but lets see if they call me."