Simple.
Coz we don’t have another right back who isn’t completely and utterly shite.
Robertson has been superb at left back so no fair to drop him.
Tierney didn’t have his best game last night, but he’s played just fine at right back in the past.
Until he got injured, Calum Paterson was starting to look the part, but for the time being unfortunately Tierney is the best option IMO