The midfielder is being linked with a move to the Premier League after an excellent start to the season with the Bluebirds, his goal at Coventry in midweek his fourth in four games and helping Cardiff move into the top three.
Steve Cotterill brings his improving Reds to South Wales this weekend and Whittingham told the South Wales Echo: "Forest are a really good Championship team, they are solid and they have picked up a bit in the past few weeks.
"It's going to be a hard game as most are in the Championship."
Check out the form guide for the match by clicking the link below.