When I heard that McCain liked him, I was worried....
He probably never figured McMaster would be back in any official capacity so no harm done by adding to his rep as a maverick by saying he liked him.
McMaster just has to make one realistic statement of fact that disagrees with McCain's lies and BS for McCain to start slandering him and talking trash about him.
FWIW, I'm not all that worried about McCain.
Folks in his age group have a lot of accidents.