Rangers should have signed Greg Stewart in January, according to Stephen O'Donnell.
Steven Gerrard's Rangers beat Scottish Premiership rivals 4-2 at Pittodrie on Wednesday thanks to two goals from Alfredo Morelos.
Sunderland were reportedly keen on the eight goal, seven assist forward.
Sunderland are reportedly keen to rival Aberdeen for Greg Stewart.
Former Rangers youngster Greg Stewart returned to stun his former club in the Scottish Premiership draw against Kilmarnock on Wednesday at Ibrox.
Greg Stewart failed to score a single Championship goal for Birmingham, but he has rediscovered his form in the Scottish Premiership with Kilmarnock.
Both players spent last season away from Birmingham City.
The Aberdeen boss said he is keen to continue working with both Ryan Christie and Greg Stewart
Hearts and Hibs are interested in rescuing Greg Stewart after a disastrous season at Birmingham City - but Rangers are nowhere to be seen.
Dundee talisman Greg Stewart was linked with Rangers last summer - but he's hardly set the world alight since moving to Birmingham City instead.