Josh Cullen emerged from his first competitive start for the Republic of Ireland having seen exactly what is required to make it at senior international level.

The 24-year-old midfielder got the nod at the Rajko Mitic Stadium in Belgrade on Wednesday evening as Ireland launched their World Cup qualifying campaign with a 3-2 Group A defeat at the hands of Serbia.

That gave him an opportunity to witness at close hand the individual brilliance of Dusan Tadic as he inspired the Serbs to a win which was secured by Aleksandar Mitrovic's double, with the striker’s introduction from the bench an indication of the standard required at the highest level.

Cullen said: "They’re a quality outfit and they’re able to make changes and bring in someone like Mitrovic off the bench.

"Of course that’s the level we need to be at and I think there were positives from the way we played and our performance at times, as well as things we know we need to get better at and can get better at.

"But I think we know the level and we just have to keep working hard as a team and as individuals to keep getting better and look forward to the next game."

That next game comes against Luxembourg on Saturday evening with the need for victory ever more pressing.

The fixture represents, on paper at least, the most winnable of the campaign, and with manager Stephen Kenny’s reign to date having amounted to three draws and six defeats, a momentum shift is urgently required.

Cullen said: "There’s no international team that you can ever underestimate, but of course, we have to now go and win on Saturday.

"It makes it a very important game, we know that, but I think we can take some positives from the performance and head into that game with some confidence.

"We know that when you’re playing these type of games, the majority of them are must-win games.

"To qualify for the World Cup, you must be winning games, so whether we had won, drawn or lost, we’d have been going into Saturday needing to win, so nothing changes there."

Cullen’s appearance in Belgrade was his fifth for Ireland and came less than six months after he took the decision to leave West Ham for Belgian side Anderlecht, a move described by Ireland boss Kenny as "brave".

The midfielder said: "To move to a massive club like Anderlecht and experience a different league and a different style of football has been beneficial for me, I feel.

"But the most important thing for any player is to be playing games, and I’ve been doing that at Anderlecht."

Follow Republic of Ireland v Luxembourg via our live blog on RTÉ.ie/sport and RTÉ News app, watch live on RTÉ2 and the RTÉ Player or listen to commentary on RTÉ Radio's Saturday Sport.