Gay, who made his Test debut earlier this summer and has held his place in the squad to face Pakistan later this month, was lining up against Middlesex in a Metro Bank One-Day Cup clash at Chester-le-Street when he was forced from the field.
The left-hander appeared to be feeling some discomfort in his right shoulder and the issue came to a head when he stroked Toby Roland-Jones for four past mid-on, immediately clutching the joint before signalling for the physio.
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Gay, who hit an unbeaten century in his previous innings against Lancashire, continued to wince in pain and eventually departed for the pavilion on nine not out seeking further assessment by the club's medical staff.
Sky Sports News understands that the ECB will assess him on Saturday and they will work with his county.
There is no specialist opening cover in the 16-strong England squad that was named this week, with the returning Ollie Pope the spare batter. He spent the majority of the last four years at number three before being dropped after the Ashes, but was recalled in the first squad since Brendon McCullum was sacked as red-ball coach.
Dan Lawrence, who was also brought back to bat at number six following the retirement of Ben Stokes, has previously batted for England in an unfamiliar role at the top of the order but is unlikely to do so again.
He averaged 20 in six innings against Sri Lanka after being called in as a short-term replacement for the injured Zak Crawley and was jettisoned from the set-up immediately afterwards.
Fleming played key role in squad selection
England men's national selector Marcus North confirmed that incumbent head coach England coach Fleming was heavily involved in the discussions as selectors attempted to ensure their philosophies were aligned moving into the future.
"As far as the dynamics of selection, it stayed consistent in terms of our system and process that we go through in terms of the wider people that influence that; Troy Cooley, David Court, Ed Barney, myself, Keesy [Rob Key], the captain, the coach," North said.
"So, there's lots of information that gets provided as part of the process. None of that's changed.
"Stephen Fleming was involved. From the moment he was appointed, he was someone that I rang and had very lengthy conversations with.
"We've got to make sure that our selection and our philosophies and thoughts around that are absolutely aligned with Stephen's. It would make no sense for us to move forward through this series and have that be contradicted to where Stephen sees us as a team moving forward."
England squad to face Pakistan
Joe Root (Yorkshire - captain)
Jofra Archer (Sussex)
Gus Atkinson (Surrey)
Shoaib Bashir (Derbyshire)
Harry Brook (Yorkshire)
Brydon Carse (Durham)
Sam Cook (Essex)
Jordan Cox (Essex)
Ben Duckett (Nottinghamshire)
Matthew Fisher (Surrey)
Emilio Gay (Durham)
Dan Lawrence (Surrey)
Ollie Pope (Surrey)
Ollie Robinson (Sussex)
Jamie Smith (Surrey)
Josh Tongue (Nottinghamshire)
England vs Pakistan (Test cricket) - August and September
- First Test (Wednesday August 19-Sunday August 23) - Headingley, Leeds (11am)
- Second Test (Thursday August 27-Monday August 31) - Lord's, London (11am)
- Third Test (Wednesday September 9-Sunday September 13) - Edgbaston, Birmingham (11am)
Watch the three-Test series between England and Pakistan live on Sky Sports Cricket from August 19. Stream cricket and more sport contract-free with NOW.
