Pompey have appealed Joe Morrell’s sending off at Oxford on Saturday.
The midfielder was harshly dismissed for a challenge on Cameron Brannagan in the opening stages of the 3-2 defeat.
That comes with a further punishment of a three-match ban, which Danny Cowley is looking to get overturned.
He is already without fellow central midfielder Shaun Williams, who is sidelined after suffering a fractured vertebrae.
The Blues boss said: “We’ve watched the moment back and have worked hard to find good footage because the side angle is probably the same view the referee had, which is impeded.
“We’re definitely appealing it and we believe we have grounds for that, but we’ll have to see what the panel decide.
“I’ve made my feelings clear to (EFL head of refereeing) Mike Jones. I’ve spoken to him more in the past three weeks than all the years before that!
“There was some bad news with Shaun last week and if we’re without Joe as well, then it leaves us incredibly light in that area – there’s no hiding place from that.
"We’re definitely appealing it and we believe we have grounds for that, but we’ll have to see what the panel decide."
“We’ve got Louis Thompson, while Ryan Tunnicliffe is still coming back to match fitness, so we’ll have to find some solutions.
“We’ve recalled Harry Jewitt-White back from his loan at Havant & Waterlooville, but Jay Mingi is not quite ready yet.
“When we’ve had injuries this season, they seem to happen in the same area. I remember playing at Gillingham with no centre-halves and we still found a way to win.
“We’ll try to find solutions within the group and I think you saw on Saturday that they’re willing to work hard and do whatever the team needs them to do.”
Pompey are also without Kieron Freeman, who was forced off at the Kassam Stadium with a head injury.
Cowley said: “He was unfortunately concussed and now has to follow the protocols, while he landed on his ankle and that’s ballooned up.
“It’s quite swollen, so that will need to be scanned. I anticipate that it will keep him out longer than the head injury.
“We’ll now have to see whether Mahlon Romeo is ready to go for Tuesday evening, but we certainly hope so.”
🏠 We return to Fratton Park on Tuesday#Pompey
— Portsmouth FC (@Pompey) February 6, 2022