The Iceland striker, who joined Cardiff City at the beginning of the season, played a key role in QPR's promotion to the Premier League in the 2010-2011 campaign.
"Robin van Persie has scored about as many goals as us this season. I think that says it all really," Green told West London Sport.
"The lack of goals has been clear for all to see and I think Andrew Johnson getting injured and missing the season hasn't helped. For me, another factor has been that the top scorer from last season was sold. That was a big decision.
"I say that with no disrespect to Jay [Bothroyd], who has also not played many games and has managed to score and play well when he has come in. But to sell someone like Helguson, who helped the club get up, was the top scorer last season and has helped Cardiff get promoted this season, I think had an effect."
Green is expected to be the number one goalkeeper at the club next season, with Brazilian stopper Julio Cesar seemingly on his way out.