But goals in the space
of five minutes for Rovers stand-in strikers Lee Thorpe and Richard Walker clinched a suprise win for the early joint leaders in League Two.
Brighton dominated from the kick-off and were rewarded in the 11th minute when Butters headed in Darren Currie's in-swinging corner from close range.
It was all one-way traffic towards the Rovers' goal until they burst into life in dramatic fashion.
Thorpe equalised in the 25th minute, gliding past Butters to score with a precise shot.
Brighton were still recovering when keeper Michels Kuipers made a brilliant one-handed stop from a downward header by Paul Trollope.
The ball broke loose for Ali Gibb to cross for Walker to convert from close range.
The contribution from the frontmen vindicated the decision by Rovers' manager Ian Atkins to rest top-scorer Junior Agogo and his regular partner Jamie Forrester.
Brighton tried to find a way back into the tie after Kuipers had parried a well-struck volley from Walker.
Albert Jarrett was narrowly off-target with a rasping, angled drive and Kevin Miller, the Rovers' captain and keeper kept out Leon Knight's diving header with his feet.
Knight had strong claims for a penalty rejected when Christian Edwards appeared to hold him back as he tried to get on the end of a cross from Currie.
Miller denied Knight again right at the death with another feet-first stop to send Rovers into the second round.