Martin O'Neill revealed that every effort is being made to bolster Celtic's squad in the January window, after they sealed their spot in the Europa League play-offs. 

O'Neill's comments came after Celtic's 4-2 victory over Utrecht on Thursday, with three goals in 19 first-half minutes ultimately enough to see off their Eredivisie opponents. 

Dani de Wit and Adrian Blake struck either side of the break to threaten a nervy finish at Celtic Park, but Auston Trusty clinched all three points with his header after 66 minutes. 

Indeed, thanks to Benjamin Nygren, Nick Viergever's own goal and Arne Engels' penalty, Celtic built the 10th quickest 3-0 lead ever in the Europa League, as well as their fastest in any major European match.

Celtic have now won five of their last six home games in European competitions (including qualifiers) against sides from the Netherlands (L1), scoring multiple goals in each of those victories.

They will now square off against Stuttgart or Ferencvaros in the next round, and O'Neill expects there to be new arrivals ahead of the play-offs.

The Celtic boss said ahead of kick-off that his team were running on empty, having only signed Julian Araujo and Tomas Cvancara on loan from Bournemouth and Borussia Monchengladbach, respectively, during the ongoing transfer window. 

"I think over the past couple of weeks we've shown great resilience. We didn't make many changes, so they've gone again. We're in the hat," O'Neill told TNT Sports. 

"The players have done brilliantly. I'm just so glad the win at Feyenoord and [the draw] last Thursday in Bologna counted for something."

On potential transfers, O'Neill added: "We're making progress and I think we can move it along now in the next couple of days.

"It's not been easy. Not just for us, for other clubs too, but perhaps Celtic fans think it's just us. We're trying to do some work, and we think we can do it.

"I can never guarantee anything, I don't think anyone can, but we're very hopeful."

Celtic's victory came at a crucial time ahead of facing Falkirk in the Scottish Premiership on Sunday, with O'Neill's side six points adrift of leaders Hearts in the table. 

And they were worthy of their win, having registered an expected goals (xG) total of 2.09 from 10 shots, compared to Utrecht's 0.43 from six attempts at Kasper Schmeichel's goal.

But it was one of Celtic's summer recruits, Nygren, who stole the show. He now has 20 goal contributions across all competitions this season (14 goals, six assists), the most by any Scottish Premiership player.

However, the overall display was what pleased Celtic skipper Callum McGregor the most, with the Scotland international lauding his team for the mentality they have shown in recent weeks, following a poor run of form that saw Wilfried Nancy sacked and O'Neill brought back for his second interim stint of the campaign. 

"We knew the objective was to win the game and get through to the play-off round," McGregor added.

"When the pressure was on, we managed to deliver results. The mentality in the group has been amazing, the boys have been hungry to dig ourselves out of the hole we were in.

"For the first 25–30 minutes, I thought we were really good."

On potential January additions, McGregor added: "We trust the club to get the players the manager and the staff want."

McGregor started his 66th game in major European competitions for Celtic, with only Billy Bremner (76) and Peter Lorimer (73) starting more among Scottish midfielders overall.