The attacking midfielder is set to miss the game due to a fractured leg.
The 25-year-old will miss the game at Hampden Park after sustaining a leg fracture during the Gers' Challenge Cup final win over Peterhead last weekend (via BBC Sport).
And while Ferguson admitted that it will be a huge blow to the Scottish Championship side's chances of progressing to the final not having Forrester available, he suggested that Law could be the man to come in as his replacement.
"The loss of Harry Forrester is a blow because it will be midfield where the game is won," he told the Record.
"Forrester is a decent foil to Barrie McKay on the other side and in the absence of that balanced width, Mark Warburton may be tempted to go with a tighter midfield five, perhaps with Nicky Law stepping into the middle alongside Andy Halliday, with Dominic Ball anchoring and Jason Holt out to the right."
While the 28-year-old spent his first two years at Ibrox as a first-team regular, his chances under Mark Warburton have been slightly less frequent.
The former Motherwell man has made 24 appearances in all competitions this season, with just two substitute appearances coming in the Scottish Cup.
And having not played since the win over Queen of the South at the end of March, it may be a risk for Warburton to throw him in on such a big stage, especially as Ferguson believes that winning the midfield battle could be crucial to the side's hopes of surprising their Old Firm rivals.