The Celtic winger had to miss recent Scotland U21 internationals and won't take part this weekend either.
The prospect played during the Hoops' last match against Rangers at Ibrox but a groin strain prevented him from taking part in matches with Scot Gemmill's U21 squad.
Beating San Marino and Croatia, the young international team have made a positive start to UEFA Under 21 Championship qualifying.
Johnston was not happy to miss out and he'll be watching from the sidelines again when club action returns this weekend.
As quoted by The Scotsman, he said: "I've just got a groin strain so I'll miss the weekend's game but I'll hopefully be back after that.
"I was buzzing when I saw the result (against Croatia) last night. I was a bit disappointed that I was out of it but I'll hopefully try to get back fit as quickly as possible."
A bright start, tougher challenges ahead
Johnston has had a fine start to the season. He certainly is proving that just because he is young he can't provide consistency and maturity in his performances.
He's scored three goals and assisted four times in 10 appearances so far, showing he can be an important part of a very talented Hoops front four that also includes Ryan Christie, James Forrest and Odsonne Edouard.
His biggest challenge in a Hoops shirt to date is on the immediate horizon though; challenging Mohamed Elyounoussi for a first-team spot.
The accomplished attacker is ready to add a bit of experienced quality to the left flank after completing a loan move from Southampton.
Johnston may find it more difficult to get opportunities as a result.
However, by taking his big chance in Neil Lennon's team when it was available, he's at least shown he can be a competent alternative option as he continues to develop and improve.
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