Fellow Hoops defender Anthony Ralston also pulled out of Steve Clarke's national team set-up, as Josh Doig was drafted in.
Robertson has revealed Tierney's injury is nothing more than a 'contact' injury, so it shouldn't keep him out for any length of time.
But the Liverpool ace admits he's gutted for his Scottish compatriot.
“KT still has a big future for Scotland, 100 percent," said the captain. "He’s a fantastic player and character.
“It’s just unfortunate that he’s turned up for camp, had a scan and it has ruled him out of both games.
“It’s disappointing for him and the squad because he’s grown into a massive part of our team.
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“It’s great to see him back for Celtic playing every week. The next step for KT is to play 90 minutes and he knows that.
“He’s getting closer. This is a contact injury so it’s not like he’s had a setback.
“He’s just been in Kazakhstan and played in an Old Firm game - and we know how tasty those games are.
“The timing is unfortunate but it won’t set him back for too long and hopefully in October he’ll be back in a Scotland jersey.”