Pompey are unsure how long Marcus Harness will be sidelined for.
A thigh strain forced the winger to limp out of the Blues' 1-1 draw at Blackpool on Saturday.
Harness was in rampant form during the Kenny Jackett's side's trip to Bloomfield Road.
He was a constant threat down the right flank and opened the scoring in the 17th minute with a neat finish. It was his third goal of the season.
However, the summer signing from Burton was forced to come off five minutes before half-time.
Despite not going down injured and receiving treatment, he was surprisingly replaced by Gareth Evans.
Harness had picked up a thigh setback, which is now likely to cause the Southend game to be postponed.
And Jackett admits Pompey don't yet know the extent of the 23-year-old's problem.
The boss said: 'It's a thigh strain and he felt it was bad enough to come off unfortunately.
'It was just during the game.
'He signalled me and then got the message across through Tom Naylor that he needed to come off.
'There hasn't been an assessment yet. It's really hard to tell with a thigh strain.
'Hopefully it is not too bad. It is a tough one to tell.'