The breakaway forward took ill after starring in the Warriors' Celtic League derby win againts Edinburgh.
Johnny will now be now sidelined for at least a month and is resigned to missing Scotland's Under-21 Six Nations campaign along with a series of crucial Glasgow games.
Johnny's dad John, the former Scotland cap and British Lion said in The Metro: "Obviously, Johnny is disappointed but at least we now know what the problem has been. After the Edinburgh win, he struggled against Borders and, clearly, the illness was working on him by then.
"He is being looked after extremely well by Glasgow doctor Gerry Haggerty and he has been told not to train for the next month.
"The experts reckon a complete rest of four weeks should see him on the road to recovery."