Chelsea head coach Frank Lampard is not yet certain what his strongest starting line up is, according to Tony Cascarino.
The Blues suffered their eighth loss of the season at St James' Park on Saturday, with a late Isaac Hayden header giving Newcastle a 1-0 win in front of their own fans.
The result saw Chelsea miss the chance to close the gap on third-placed Leicester, who also suffered a defeat at Burnley over the weekend.
Lampard's men could have put some clear daylight between themselves and Manchester United in the race for the final Champions League spot too, but they failed to make the most of their dominance against the Magpies.
The 41-year-old said Chelsea needed "to be more clinical" in the final third post-match, but Cascarino thinks he has identified a more pressing issue for his old club.
The ex-Blues striker highlighted Lampard's lack of conviction in selecting his team, particularly when it comes to who occupies the two main roles in the heart of the defence.
“I’m not convinced he totally knows who is his best XI,” Cascarino told talkSPORT. “[Callum] Hudson-Odoi’s been in, Willian’s come in, [Christian] Pulisic has come in. Mason Mount has been in and out the team, Reece James has played…
“If you look at the centre-halves, he’s had [Kurt] Zouma and [Fikayo] Tomori. But [Andreas] Christensen played alongside [Antonio] Rudiger yesterday?
“There are four centre-halves and it could be any two! And you’re not really sure that they’re his best two.
“And Christensen – you were not really sure if he was going to play a part at all because a couple of weeks ago Frank Lampard said he might actually go and play elsewhere.
“So it’s a really weird scenario where Lampard does not know his best XI.”
Lampard has already promised not to make any "knee-jerk" signings during the January transfer window, but Chelsea continue to be linked with a number of top players amid a worrying slump in form.
Goal has learned that the Blues manager is a keen admirer of Leicester full-back Ben Chilwell, though he is unlikely to leave the King Power Stadium this month.
Lyon striker Moussa Dembele has also been touted for a switch to Stamford Bridge, but his current employers have publicly insisted that he will not be sold.
Chelsea are now looking ahead to a crucial home fixture against Arsenal on Tuesday, which comes five days before an FA Cup fourth-round tie against Hull City.