Mark Natta wasn't exactly sure of the reasons for the Newcastle Jets' flat performance at the back against the Mariners in a 3-1 loss on Saturday. But the defender was sure "it won't happen again" as Newcastle prepare to host under-pressure defending A-League premiers Melbourne City on Sunday, and Natta himself tries to enhance his Paris Olympics chances. Natta, who turned 21 on Tuesday, and left-back Lucas Mauragis, 22, returned from Olyroos duties late last week before playing 77 and 63 minutes respectively at Gosford. Storm Roux scored in first-half stoppage time and William Wilson in the 57th minute, both on the Jets' left edge, to set up the win and the 2022-23 champions' first points of the season. The lead was deserved after the Mariners dominated chances created against a sluggish Newcastle side. Jets coach Rob Stanton took responsibility for the lacklustre display, saying he trained the squad too hard during the international break. Natta said the players had to take their share of responsibility as well. "The coach can only do so much on game day," Natta said. "We're the 11 players out there ... working and playing the game, so I think we have to take the most responsibility on game day. "I'm not too sure exactly what it was. I think we all came out a little bit flat. I'm not sure the occasion might have got to us, but I think it's definitely something we'll fix during the week and it won't happen again." Natta and Mauragis were part of a youthful backline which also featured Phillip Cancar, 22, and Dane Ingham, 24. And while Newcastle have shown attacking promise this season, their defence remains a concern. Five games in, the Jets sit seventh on five points but hold the equal-worst defensive record of 12 goals conceded, alongside City, who have just four points. A regular starter last season at centre-back, Natta said defensive structure had been a focus in recent weeks at Jets training and he welcomed his added responsibility in the lineup. "I think I'm very fortunate to have played as many games as I have  at my age and be in more of a responsible role at the back," he said. "It's something I want to do and I'm happy to do and I think I'm getting better as the games have gone." Another focus for Natta is making the Olyroos side for the U-23 Asian Cup in Qatar in April. The top three nations at the tournament qualify for the 2024 Olympics. Australia were on Tuesday drawn in group A along with Qatar, Jordan and Indonesia. Natta and Mauragis played in a 2-1 win over Qatar in a friendly last week in Saudi Arabia. "Playing for your national team is the dream and the end goal for me, so hopefully have a good season with the Jets and play my game, and to get called up would be a buzz," he said. "I think we did pretty well [against Qatar]. We controlled the game pretty good ... we defended pretty well. They got one lucky chance, to be honest, and they took it."