Andy Awford has admitted Jack Whatmough’s on-going injury issues are causing concern.
The teenager was forced off against Tranmere after 27 minutes on Saturday, with Ben Chorley replacing him.
The Pompey boss afterwards revealed it was the recurrence of a knee problem for the defender.
The Gosport youngster was only four games into a return following a hamstring pull at York.
And Awford revealed his worries over the continued unavailability of the central defender.
He said: ‘Jack has a sore knee again. It has recurred and we will have to get to the bottom of that.
‘He came off at Plymouth with cramp, which is not related to what he had on Saturday, but he does have a problem with his knees.
‘I don’t know too much, you would have to speak to the physios. I don’t know the ins and outs of it but it is something we need to monitor on a daily basis.
‘On Saturday it was too painful for him to continue.
‘That wasn’t anything to do with cramp – that’s an actual injury we are aware of and trying to maintain as best we can.
‘Jack was fully fit at the beginning of the week – then when he was out there something has gone wrong.
‘He’s not had a huge run of games this season, that has been disappointing for him. He’s had a stop-start season.’
Meanwhile, Awford is hoping for positive news on Ryan Taylor’s injury.
The striker came off in the 56th minute at Prenton Park after rolling his ankle.
The Pompey boss added: ‘Ryan went up for a header and went over on his ankle and, obviously, couldn’t carry on. He tried to but had to come off.
‘We won’t know that until we get back in and assess it.’