Martin O’Neill admitted Celtic have a defensive issue to fix after Thursday night’s 4-1 defeat to Benfica in Lisbon.

The Hoops were 3-0 down by half-time at the Estádio da Luz, with Jhon Durán scoring twice for the hosts and Alexander Bah also finding the net.

Catch up on Martin O'Neill's return as Celtic manager in full.

Benfica found space too easily in the first half, especially for the second goal, when Bah was left free at the back post to volley beyond Viljami Sinisalo.

Speaking to BBC Sport after the match, O’Neill was asked whether he was concerned by how easily Benfica cut through Celtic at times.

He said: “Yes, I do. I think that they’re getting in rather easily.

“It’s just one cross. We’re coming out instead of defending it, defending the space or defending the player properly.

“That’s an actual concern.

“Hopefully, we can eradicate that during the course of the season.”

That was the bluntest part of O’Neill’s post-match reaction.

He praised Celtic’s second-half response, but he did not try to dress up the defensive problems that left his side with too much to do.

The early penalty gave Benfica the perfect start, but Celtic’s biggest frustration will be what came after it.

Bah’s goal was preventable, and Benfica’s third also came from slack play before Jhon Durán squeezed a finish beyond Sinisalo.

Speaking to TNT Sports, O’Neill was also critical of the way Celtic handled the second goal.

He said: “One is just a little ding in a cross, and we’re coming out instead of defending it properly.

“And that’s really poor from our viewpoint.

“So you can’t afford to do that.

“I don’t think you can afford to do that in any game, never mind a game of this magnitude.”

That is the concern going into Sunday’s league match against Hearts.

O’Neill can take something from the way his side responded after the break.

He cannot ignore how easily Benfica built a three-goal lead before Celtic started playing with enough urgency.

The next step has to be cleaner defending, because Celtic are giving themselves too much work to do before their attacking players get a chance to change games.