The original GOP plan was for a showdown vote on Wednesday to begin debate on the health care measure - but if McCain is back in Arizona, that would seem unlikely to go forward, as GOP leaders need every single vote they can get to keep the health bill alive.
The news about McCain came a few hours after President Trump had used his Saturday radio address to again put public pressure on reluctant GOP Senators to support the Republican health care bill.
As of now, Sen. Susan Collins (R-ME) and Sen. Rand Paul (R-KY) have said they will not vote to start debate on the bill.
Four other more moderate Senators - Portman of Ohio, Murkowski of Alaska, Capito of West Virginia and Heller of Nevada remain undecided about their votes.
As for McCain, there were plenty of well wishes for the Senator from Arizona.
"Looking forward to a speedy recovery, John," said Sen. Brian Schatz (D-HI).
"Praying for a speedy recovery for my friend," said Senate Democratic Leader Charles Schumer.