Celtic captain Scott Brown is expected to be out for around three months with a hamstring injury, BBC Scotland has learned.
The midfielder was carried off a few minutes into his side's 1-1 friendly draw with Rapid Vienna at the weekend.
As well as missing Celtic's Champions League qualifiers, Brown is also likely to miss at least one of Scotland's Euro 2016 qualifiers.
Further news is expected after Tuesday's friendly against LSK Linz.
Celtic travel to Iceland to take on KR Reykjavik on 15 July in the second round of Champions League qualifying, with the return leg the following week at Murrayfield.
The Scottish champions then kick off the defence of their domestic title on 13 August away to St Johnstone.
And with Scotland facing Germany away on 7 September in their first Euro 2016 qualifier, Brown looks set to be unavailable.