Pompey have four senior players who are comfortable playing at centre-back.
A serious knee injury suffered by Jack Whatmough has reduced that number, while Christian Burgess picked up a calf problem earlier in the week.
Nathan Thompson has now booked his place in the treatment room following a nasty collision with team-mate Drew Talbot that saw him stretchered off with concussion.
It leaves Matt Clarke as the last man standing and he had to be partnered by striker Oliver Hawkins for the second half of a 2-1 defeat to Oldham.
The defender said: “It was a disappointing day all round, with the goals we conceded and then losing Nathan as well, which was a kick in the teeth.
“Someone perhaps had to take charge, but it was just one of those horrible incidents that you don’t want to see anyone go through.
"Oli’s got the size and that was probably the factor in why he moved back, but I thought he adapted really well."
“With Burge not being available, it’s not an issue because Nathan comes in. But now it looks like he’ll also have a spell out.
“Oli’s got the size and that was probably the factor in why he moved back, but I thought he adapted really well.
“The gaffer will decide what we have to do next, we’ll work on it and try to be a stronger unit for the next game.
“I’ve got to look after myself and do whatever I can to stay fit, but I don’t want to jinx myself by commenting too much on that!”
A brace from former Fratton loanee Eoin Doyle gave the visitors a commanding lead before Brett Pitman grabbed a late consolation.
Clarke said: “We left ourselves with far too much to do after going 2-0 down. The big difference on Tuesday night was that we got the first goal
“I think we just need to stamp our authority on games a bit more, so we don’t give ourselves a mountain to climb.”