Emilio Gay has made three half-centuries in his first summer as a Test opener. England's Test opener does not necessarily have to be a specialist at the top of the order, according to Marcus Trescothick.

England have a decision to make over how to accommodate Jacob Bethell when he returns from a knee injury for the tour of South Africa in December. Emilio Gay, in his first summer as a Test cricketer, has opened alongside Ben Duckett. Gay has made three half-centuries, though he ends the season with an average below 25. Duckett, one of England's senior players, made the team's only century of the summer, against New Zealand at Trent Bridge in June. However, the 31-year-old did not reach 20 in the three-match series against Pakistan and averages less than 24 since the beginning of the Ashes.

Jordan Cox has impressed as Bethell's replacement at number three. After finally making his debut this summer, Cox has averaged more than 42 and opened in first-class cricket early in his career. Bethell has been touted as a potential opener and has moved up to open in England's one-day team.

The other new member of England's top six for the Pakistan series, Dan Lawrence, may be exempt from conversations over who is squeezed out because of the off-spin he offers, though Bethell's return would also bring some left-arm spin.

Trescothick has been England's interim coach for the Pakistan series and will now hand the reins to New Zealander Stephen Fleming, who is due to travel to the UK in the coming days to begin planning for the trip to South Africa. "You generally want your best top seven, however that looks," said Trescothick. "Who that's going to be, I don't know. That's for someone else to work out."

Trescothick is one of England's most successful Test openers and was part of the team that famously won the 2005 Ashes. The former Somerset man believes the game is different to when he ended his 76-Test career in 2006. "I reckon times have changed a little bit," said the 50-year-old. "I think you look at your best batting line-up, however that fits up. If that's with a specialist opener, great. If not, I think you can plan around it in this day and age. Players adapt pretty quickly because they're chopping and changing formats."

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Fleming was appointed England's new Test coach in July, yet he missed the 3-0 series win against Pakistan. The former New Zealand captain will now arrive for meetings with the England hierarchy. He is expected to watch some of England's white-ball series against Sri Lanka, with the ODI and T20 teams still led by Fleming's fellow Kiwi and good friend Brendon McCullum. Fleming could also visit some of the counties.

When McCullum was in charge of both the Test and white-ball teams, he kept the same backroom team across formats, including Trescothick as an assistant. However, it now looks likely that the staff will be reorganised. Former Australia batter Mike Hussey said he was approached by Fleming, though it has been reported down under that Hussey has declined.

Trescothick said he has not got clarity on his future role. He confirmed he has a preference but declined to reveal it. "It's probably best I don't mention it at the moment because it may not be the case of what happens," he said. "I love doing both formats. It's brilliant and I've been privileged to be in the position of assistant coach."

Trescothick will be joined on England's staff by former team-mate Kevin Pietersen, who has been appointed as a consultant for the white-ball team. Pietersen, one of the most flamboyant and innovative batters to play for England, returns to the national team 12 years after he was effectively sacked following the 5-0 Ashes defeat in 2013-14. The 46-year-old's first day back in the England set-up is on Sunday, joining preparations for the first T20 against Sri Lanka in Southampton on Tuesday.

"We've spoken already, to plan a little bit," said Trescothick. "I gave him a couple of things that I wanted him to look at, to get his advice on. He is buzzing. Putting back on the shirt or the tracksuit, he is buzzing for that opportunity. Having his presence around and the knowledge he has about batting – he sees batting in a different light to many different people. If we can tap into that, it's only going to benefit the team."

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