The 31-year-old midfielder broke down while warming up as a substitute during last week's win at St Mirren and will need surgery on a knee problem.
Wright told the club's website: "Our season has already been marred by freakish, 'non-contact', serious injuries. On a disappointing note, it looks like Gary may need a similar operation to Steven MacLean.
"That will make seven serious injuries to key players throughout the season, but I also feel that it highlights the strength of the squad and the level of achievement gaining top six has been."
MacLean missed four months of the season with a knee injury before making a goalscoring comeback against Motherwell in February.
McDonald has made 27 league appearances this season and is under contract at McDiarmid Park until the end of next season.