The Lions: Manu Tuilagi and George North knocks give Warren Gatland a headache

Centre and wing are doubts for Test opener so Zebo to make debut against Waratahs

The pressure is intensifying, the injuries are mounting and the clock is ticking: the Lions have only 160 minutes of playing time left to them before the big decisions on form and fitness must be made. Warren Gatland, the head coach, indicated that two important backs – the Test front-runner George North and the prime "impact" player Manu Tuilagi – might be struggling to make the cut for the first meeting with the Wallabies a week on Saturday. If it is not yet a case of panic stations, rear ends are definitely beginning to squeak.

When Gatland announced his side for Saturday's meeting with a New South Wales Waratahs outfit that suddenly looks a lot more dangerous than it did – the unexpected release of two senior players from the Wallaby camp, the centre Rob Horne and the flanker Dave Dennis, will beef up the home side significantly – he stressed that selection for the Brisbane Test had not been discussed.

But there was at least a whiff of decisive intent about the shaping of the pack, with the two form props in the party, Mako Vunipola and Adam Jones, and the most experienced locks, Alun Wyn Jones and Paul O'Connell, forming the body of the tight-forward unit. With Jamie Heaslip, the Irish No 8, holding his place after starting the midweek runaround against the wannabes and might-have-beens of the Combined Country XV in Newcastle, the men currently in possession of the shirts have every chance of driving home their advantages.

It is a very different story behind the scrum, however. North picked up a hamstring strain on Tuesday and was not considered for the Waratahs fixture; Tuilagi has yet to recover from the shoulder injury he suffered early in last weekend's victory over the Queensland Reds; and Brian O'Driscoll, the squad's kingpin figure, has a minor groin problem. While the latter is not thought to be a serious concern, the others are well worth worrying about.

"Brian could play against the Waratahs but if he gets another knock he could miss the Test, so we're not going to risk that," the coach explained. "He's been great on tour, but he's 34 now and we have to be smart in managing him. We need to make sure he's fit for when he's next required." Then came the killer coda. "There are also one or two who are a little bit sore and who may be out of contention for Brisbane because they won't be fit enough early in the week."

All this explains why Simon Zebo, the "now for something completely different" wing from Ireland, has been thrown into the Waratahs game within days of joining the squad as cover for his broken countryman Tommy Bowe, and why the versatile goal-kicking midfielder Billy Twelvetrees has been summoned from the England tour of Argentina. Both are blessed with rich talent, but a Lions Test at this stage of their careers might easily frazzle them. Gatland needs his medical staff to get North right for next weekend. Badly.

Zebo's try against Wales on the opening weekend of the Six Nations in February was quite something – he may have been the first wing in union history to pull off a genuinely convincing impersonation of Diego Maradona in completing a touchdown – marked him out as something special, and he was marginally unlucky to miss out on a place in Gatland's original selection. Now, with Bowe recovering from surgery on a busted hand and Alex Cuthbert of Wales in unconvincing form, he has the chance to make a name for himself.

"Simon has a personality that will flourish within this Lions party," the coach said. "He's in a no-lose situation here. He can go out there against the Waratahs and enjoy himself, and if he plays well he puts his hand up, doesn't he? I think it will be great for him."

Much to the tourists' relief, Jonny Sexton and Owen Farrell, the Lions' only specialist outside-halves, have been judged fit to play a part in proceedings at the Sydney Football Stadium. But Farrell still appeared to be struggling with his "dead" leg during kicking practice – a sign that the rigours of the tour are taking their toll. According to Gatland, the physicality levels are up on four years ago, when the Lions visited South Africa.

Robbie Deans' unexpected release of Horne and Dennis for this weekend's contest – the Wallaby coach has also made the prop Scot Sio and the flanker Peter Kimlin available to the Brumbies for the match in Canberra on Tuesday – is not designed to soften things up for the Lions. "It's a good opportunity for those players to have a run against us and go back into camp with a bit of information for the Wallabies as well," Gatland remarked with a wry smile.

Anxious Aussies hand out hats

The Aussies are trying to minimise the effect of the massed ranks of red-shirted Lions fans during the series – a strong contributing factor to the tourists' victory in Brisbane 12 years ago.

This time, thousands of gold- coloured safari hats, known as "lion-hunting pith helmets", will be handed out to the home crowd. "We were caught unawares in 2001, when the 'sea of red' was a dominant feature in the Gabba Test," admitted Bill Pulver, the CEO of the Australian Rugby Union. "The Wallabies felt like they were playing away and we don't want a repeat of that in 2013." It seems Gatland and company are not the only ones in "squeaky bum" territory.