Site of his maiden Grand Slam triumph, Cilic has suffered all season with this injury and will now miss another part of it.
It is yet another setback which means he will miss all tournaments over the next month and likely will aim to get back ready for the back end of the season and the Australian Open.
The 34-year-old won the title back in 2014 in his finest hour, but he will now certainly drop well outside the World's top 100 after this setback.
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"Unfortunately, not great news these days. Had a setback with my knee which will postpone my return on the tour and I will have to miss the US swing of tournaments," he wrote on Instagram.
"While I am extremely disappointed to be missing another part of the season, I believe better days are ahead & will keep working hard to get there. Thank you to everyone for their continued support."