Rangers headed to Celtic Park on Wednesday night sitting top of the Scottish Premiership table and with a two-point lead over Celtic.
The Gers were quickly on the backfoot at Celtic Park as the Bhoys started quickly and scored through Reo Hatate with five minutes on the clock.
Celtic were dominant and scored a further two goals to go in 3-0 ahead at the break and in cruise control.
There were no goals in the second period, but Celtic were never in danger and are now top of the league.
Miller thinks the defeat will be a hugely damaging episode for Rangers and will now need to bring about a mentality shift at Ibrox as they are the team chasing.
“Off the back of the two draws, this is going to be very damaging”, Miller said on BBC Sportsound.
“Rangers are playing catch-up now. It’s a different mentality.
“You have to beat Ross County away, not concede three goals. And they were outplayed at Pittodrie.
“Rangers have to find answers and find them quickly”, he added.
Rangers are next in action against Hearts and boss Giovanni van Bronckhorst will be looking for his side to bounce back quickly.
