If there was ever a former player who had one eye on Celtic’s game with Rangers on Sunday, it’s Kieran Tierney. The Arsenal defender is currently on loan at Real Sociedad andenjoying a run in the Spanish club‘s first team. Having suffered a hamstring injury at the start of the season, the former Celtic man returned to action in February and in the four games he has appeared in, Sociedad have lost one and were victorious in their last three La Liga matches. However, that hasn’t stopped Tierney from keeping an eye on his boyhood heroes and admits that the Ibrox game was ‘one of the best’ he has …