The 25-year-old took a ball to his face from close range when the Hibs’ defender Rocky Bushiri tried to make a clearance.
After receiving treatment from the Hoops’ medical team, Johnston initially returned to the field to continue play.
However, he was back down again after a minute and manager Brendan Rodgers replaced him with Anthony Ralston on 25 minutes.
The Canadian footballer is believed to have suffered from a suspected concussion, although it is yet to be confirmed.
Johnson’s injury is a big blow for Celtic as he has been a key member of the team this season.
After missing the first two league games due to an ankle injury, he started the following seven matches in the Premiership.
He averaged an impressive 2.4 tackles, 1.6 clearances and 1.7 key passes per game in the Scottish top-flight this term (as per WhoScored).
The Bhoys already have Reo Hatate sidelined with a hamstring injury.
The Japanese central midfielder received a knock during the Hoops’ 2-2 draw with Atletico Madrid in the Champions League on Wednesday.
Rodgers later gave a worrying update about Hatate, saying the midfielder’s injury looks bad.
Celtic are currently at the top of the Premiership standings after winning eight of their ten matches.
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