Why it's a critical fortnight for the Caltex Socceroos

The next fortnight promises to be critical to the Socceroos' future with a few selection questions still unanswered ahead of the next round of World Cup qualifiers.

Coach Ange Postecoglou deserves plenty of credit for improving the depth available to Australia's senior national team but there remain some problem areas in his line-up where players need to lock down a starting position.

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Positive results against England on Friday and in the two-game series versus Greece in June will certainly give Socceroos fans some joy but the long-term success of Postecoglou's team could be defined by how the following three questions are answered.

Massimo Luongo

Who are the Socceroos' leading full-backs?

In the last 12 months, Postecoglou has used eight different players in the full-back positions but none of them have made an undeniable case for a regular starting spot.

In eight matches, no full-back has started more than three games.

After the 2015 Asian Cup, Ivan Franjic appeared to have the right-back position sewn up only for injuries to push him back down the pecking order, while left-back looks to be Brad Smith's to lose with the Liverpool man slowly making an impact in the Premier League.

Brad Smith in action for Liverpool in the Premier League.

The main concern for Postecoglou is none of his options seem truly comfortable in both major tasks of the modern full-back.

Ryan McGowan, James Meredith, Josh Risdon and Tarek Elrich are arguably the most natural defenders of the current crop but less impressive in the forward third.

Franjic, Aziz Behich, Smith and Jason Davidson enjoy overlapping down the wing but can be found out in defence.

If Postecoglou's high-octane style is to work against the world's best, he will need to find two rock-solid full-backs.

Does Postecoglou have a post-Cahill plan?

Australia's all-time leading goal-scorer Tim Cahill will miss the England clash in Sunderland as the match falls outside FIFA's international windows and he is required in the Chinese Super League (CSL) with Hangzhou Greentown.

Tim Cahill returns to the Caltex Socceroos squad to face Greece.

It seems every time critics question Cahill's longevity the talismanic attacker finds another gear, so it would be foolish to rule him out of leading Australia to - and playing in - the 2018 World Cup.

But the 36-year-old, who will be involved in the Socceroos' squad to face Greece, has had a slow start to his Hangzhou stint with just two CSL goals - both from the penalty spot - in 10 matches.

Postecoglou is pragmatic enough to know he cannot rely on Cahill forever but the 50-year-old coach has yet to discover a reliable replacement up front.

Tomi Juric is considered by many as Cahill's heir apparent but injuries have thwarted his momentum with Roda JC in his first season in the Eredivisie, while Apo Giannou is in a similar mould as the ex-Western Sydney Wanderers centre-forward.

Brisbane Roar striker Jamie Maclaren was the most prolific Australian in the A-League in 2015-16 and it would be intriguing to see him get a run versus either England or Greece.

Jamie Maclaren celebrates after scoring for the Olyroos at the 2016 AFC U-23 Championship.

Another option for Postecoglou would be to tinker with his formation up front to see whether a front two - perhaps Juric and Maclaren together - could be the way forward in the post-Cahill era.

What is Australia's recipe for midfield success?

The Postecoglou era has seen the rise of two playmakers for the Socceroos - Massimo Luongo at the Asian Cup and Aaron Mooy in the World Cup qualifiers.

With Tom Rogic also making an impact at international level, Australia are blessed with riches in the attacking midfield area.

Tom Rogic unleashes a shot against Jordan at Allianz Stadium.

The Socceroos' midfield depth doesn't stop there with captain Mile Jedinak and stalwart Mark Milligan the perfect screening duo in front of the back four, while the latter can also push forward if Postecoglou only wants one holder.

It provides Postecoglou plenty of options in the middle of the park - something most coaches would love to have.

But in the most critical matches - think Japan home and away in the next stage of qualifying for Russia 2018 - who will the Socceroos' boss turn to?

Postecoglou often prefers some variant of a 4-3-3 formation with Cahill, Mat Leckie and Robbie Kruse in the attacking positions, which leaves limited spaces for midfielders.

Australia's upcoming three matches would be the perfect platform to make a statement.