The Australian has had his red card from the game with St Mirren downgraded.
Goals from Nicky Clark and Kenny Miller did the job as a Haris Vuckic inspired Rangers took the win at Ibrox.
They now travel to Easter Road with an imposing lead on the tie. If they can hold out they will take on Motherwell in the final - to decide who will fill the final spot in the SPL next season.
Motherwell have now been handed a massive boost ahead of the game, as a former Celtic hero has just been cleared to feature.
Scott McDonald spent three years with Celtic between 2007 and 2010. After that he spent a spell with Rangers arch-enemies Celtic. McDonald spent three years at Parkhead – scoring 64 times in 128 appearances before heading to Middlesbrough.
In February he joined Motherwell for a second stint at the club, after Millwall released the Australian in January.
He was charged with keeping the club in the SPL, but despite five goals in 11 appearances, they are still on the precipice.
It then looked like he would miss the play-off final. He was sent off against St Mirren on the final day of the season, but that has now been rescinded, as reported by Sky Sports.
So this former Celtic hero might just stand in the way of Rangers' glorious return to the top-flight.