Pompey are confident George Thorne will be fit for the south coast derby.
The teenage midfielder limped out of Saturday’s disappointing 5-1 defeat to Burnley in the 81st minute.
He later left Fratton Park on crutches with his left ankle strapped up.
The prospect of losing one of their most impressive performers of late is an alarming one for the Blues.
Thorne was superb during the opening 45 minutes against the Clarets.
Now the 19-year-old West Brom loanee has fitness concerns heading into the crucial trip to St Mary’s on Saturday.
But Michael Appleton is hopeful the problem is not as serious as it would suggest.
He said: ‘George has rolled his ankle, he has tweaked it a little bit and we are not sure how serious it is.
‘We will have a look in the next 24 hours and see how it is. I don’t think it is too bad, luckily.
‘Hopefully he will be okay for the Southampton game, he has a week to try to get it right.’
Thorne is currently in his second loan spell at Pompey.
He has impressed during his 13 appearances so far this season, emerging as a key figure in the centre of midfield.
The England under-18 international was outstanding against Hull in the 2-0 midweek win and Pompey will be desperately hoping he will be declared fit to face Southampton.
Thorne is on loan until the end of the season.