Blackburn has missed practice all this week and was wearing a cast Friday. He said it was "highly unlikely" he would be ready for New England.
"He's struggling with it," coach Ron Rivera said. "More than likely he won't [play]."
Blackburn's injury is one reason the Panthers on Wednesday signed Dan Connor, released from injured reserve by the New York Giants. Connor was with the team in Rivera's first season in 2011, so he is familiar with the system.
"Feeling pretty comfortable about that," Rivera said.
Rivera also is comfortable with Klein, a fifth-round pick out of Iowa State. Klein more than held his own against San Francisco with five tackles, two tackles for loss and a sack.
"He stayed with it," said Blackburn, who came to Carolina from the Giants during the offseason. "The moment wasn't too big for him. He was able to go out there and execute. He didn't try to do too much and stayed within himself."
Blackburn, who he re-injured the foot trying to make a tackle on special teams, said he got his first opportunity the same way against the Chiefs in 2005.
"Just like that, I got thrown in," he recalled. I had a pretty good game just like that. It's always good when you give an opportunity to a young guy and he takes advantage of it."
Backup tight end Ben Hartsock also was on the injury report with a sore knee. Rivera remains optimistic he will be ready by Monday.