Full-back Griffin signed on loan from Stoke for the remainder of the season on Tuesday while Icelandic striker Thorvaldsson has received international clearance to complete his loan from Esbjerg.
The duo were not eligible for the famous 2-1 FA Cup third-round replay win at Liverpool on Wednesday night but will both travel to the City Ground.
Griffin is set to start at right-back in place of converted midfielder Brynjar Gunnarsson while Thorvaldsson will have to settle for a place on the bench.
Shane Long will hope for a start after he came on to head the winner at Anfield but interim boss Brian McDermott may stick with the combination of Grzegorz Rasiak and Simon Church in attack.
Noel Hunt and Chris Armstrong (both knee) remain the Royals' only injury worries.