That is according to pundit and Newcastle legend Alan Shearer.
The Blues have struggled for form of late, winning just one of their last seven games in normal time.
Antonio Conte's side welcome Newcastle to Stamford Bridge this afternoon, as they look to secure their place in the fifth round.
Chelsea struggled against Norwich last time out, needing penalties to progress against the Championship outfit.
And Shearer believes a repeat performance could see Newcastle pull off an upset.
"It's an opportunity [for Newcastle]," he told Coral.
Chelsea's likely line-up to face Newcastle at home
Chelsea FC
“But if Chelsea play a full strength team then it will be a very tough day for Newcastle”
Alan Shearer
"I watched the replay, Chelsea vs Norwich at Stamford Bridge.
"And Chelsea were slightly fortunate, getting through on penalties.
"It could have been different.
"So if that Chelsea turns up, the one that weren't good enough to beat Norwich over two games in open play, then Newcastle will have an opportunity."
Conte admitted key players will be absent for his side this afternoon.
Willian, Cesc Fabregas and Alvaro Morata are all doubtful for the clash.
Meanwhile, Rafa Benitez played a full strength side in the previous round against Luton.
And Newcastle fans will be expecting the same today.
Getty
"It was refreshing that Rafa played a stronger team in the previous round against Luton and they got the right result," Shearer continued.
"Over previous years that hasn't been the case with Newcastle.
"So hopefully he'll play a strong team again.
"And as I said, if that Chelsea turns up then Newcastle could have an opportunity.
"But if Chelsea play a full strength team then it will be a very tough day for Newcastle."