The 21-year-old missed the Hoops' Scottish League Cup semi-final defeat to Ross County.
The former Dinamo Zagreb man sat out Sunday's Scottish League Cup semi-final defeat to Ross County, and had to watch as the Hoops fell to a surprise defeat, with the man he has often featured alongside at the heart of the defence, Efe Ambrose, being sent off after just 14 minutes at Hampden Park.
While Ronny Deila completed a deal to sign Erik Sviatchenko this month, the news of Simunovic's injury will surely be a huge blow to the Celtic boss. While their hopes of retaining the League Cup are over, they are still in the mix to win the Scottish Cup and have a commanding lead in the Premiership.
However, Deila would have been hoping to have had the 21-year-old available in the coming weeks. Their next game in particular, could have a major say in the destination of the league title, as they are set to come up against second place side Aberdeen - who currently sit six points off the table's summit - at Pittodrie.
While the early stages of Simunovic's career at Parkhead were often blighted by injury (via The Scottish Sun), he had started to establish himself as a regular member of the side in the last couple of months.
Before missing the defeat to Ross County, the Croatian had been involved in the side's run of five consecutive victories. And Deila will be desperate to ensure that Simunovic's absence does not result in further defeats as that may potentially leave the door open for Aberdeen to make the title race a little more interesting.