Williams suffered collapsed lung during Mexico draw
The 33-year-old was substituted in the 21st minute of the 0-0 draw at the Rose Bowl in California after a collision with striker Javier Hernandez.
Williams was taken to hospital where tests showed he had broken two ribs, which caused his lung to become punctured and collapse.
The former Swansea City centre-back thanked staff for their treatment during and after surgery and is now looking forward to returning home.
Writing on his official Facebook page, Williams said: "Crazy couple [of] weeks. Great to be back in a Wales shirt and with the lads, however after coming off after 20 minutes with multiple breaks on two ribs, I went [to] hospital and found out I had punctured my left lung which led to it collapsing.
"I would like to say a huge thank you to all the staff at Ronald Reagan UCLA hospital in Beverly Hills for looking after me and making sure surgery went as comfortable as possible.
"Look forward to flying home and resting up now before the new season!"
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Crazy couple weeks.. great to be back in a FA Wales shirt with the lads, however after coming off after 20 mins with...
Posted by Ashley Williams on Saturday, 2 June 2018