Charlie Mulgrew believes Liam Scales may have been carrying a knock after the defender was left on the bench during Celtic’s 4-1 defeat to Benfica.

The Irishman has been a regular under different managers, and even when he has not been at his best, he has usually been trusted in big games because of his aerial strength and defensive discipline.

Keep track of every knock and return with our Celtic injury news.

Martin O’Neill instead started with Dane Murray and Auston Trusty in central defence, with Kieran Tierney at left-back and Colby Donovan on the right.

It turned into a difficult night for Celtic’s back line.

Benfica were 3-0 up at half-time after Jhon Durán scored twice and Alexander Bah also found space at the back post to volley home.

Speaking on Premier Sports Ireland, Mulgrew suggested Scales’ absence from the pitch may have been down to fitness.

He said: “Scales, you can only assume he’s got a knock and is being rested. Because I think you see how big a player he is for Celtic. It’s not always appreciated but he is somebody who heads the ball in the box and defends well.”

That point stood out during the first half.

Celtic looked too easy to open up, and Benfica’s second goal in particular left questions about positioning and who was attacking the ball.

Bah had time at the far post to finish after Gianluca Prestianni’s delivery, with Sebastian Tounekti losing track of the run.

Mulgrew also felt Scales may have dealt better with Benfica’s third goal, when Jhon Durán squeezed a finish beyond Viljami Sinisalo before half-time.

He said: “The Trusty one, I think he should have blocked the goal just before half-time. I think Scales blocks that, gets his leg in and stops the line of the ball.”

That is the kind of defending Scales is often judged on.

He is not always the cleanest player in possession, but he does attack crosses, puts his body in the way, and gives Celtic a more natural penalty-box defender when games become direct or stretched.

Celtic looked better after the break in Lisbon, but the damage had already been done, and the manager cannot afford another first half where his side look so open.

If Scales was rested because of a knock, Celtic will hope it is nothing serious.

If he was simply left out on selection grounds, Mulgrew’s comments show why that call will now be questioned after another poor defensive night in Europe.