Chelsea have slumped to consecutive defeats this week at home and in Europe.
"I spoke over the summer about the urgency of buying a replacement for Didier Drogba and I thought the best player out there was Benteke," Cascarino told talkSPORT.
"I felt that would have been a massive statement. He's the type of the player they needed in the way they play.
"The way Mourinho's teams have played over the years, he loves that physical man, and he doesn't have it. He has three forwards and doesn't know which one to pick.
"It would have made a huge difference [getting Benteke in]."