In the case of substitute Harrison, with Danny Cowley having already used his allocation from the bench, it meant the Blues had to see out the final 25 minutes with 10 men.
Downing was forced off shortly before half-time with a hamstring problem, while Harrison departed Fratton Park with his left leg in a protective boot following a challenge on his ankle.
In addition, Mnoga, who was replaced at half-time by Shaun Williams, suffered a dead leg.
Cowley told The News: ‘Paul (Downing) just can’t get a run at the moment. He’s a good specialized, he does everything properly, but his body just lets him down.
‘He’s getting recurring injuries and his body’s letting him down at the moment, I feel sorry for him.
‘With Ellis, it was a terrible tackle on him, their player has really left it on him. It’s a straight red for me.
‘I’ve had the opportunity to watch it back. I’ve got to be honest, I didn’t think it was (bad) live, so I have to be fair to the referee.
‘But when I watched it back in slow motion he went right over the ball and Ellis has been left with a swollen ankle.
‘It doesn’t look good, we will have a examine in the morning. It’s swollen, which method it’s angry, so we will have to see how it settles.’
In the case of Mnoga, the 19-year-old didn’t reappear after the interval having operated as a central midfielder.
However, Cowley was rather more optimistic over the Academy graduate, who is on loan at Bromley until January.
Pompey’s head coach additional: ‘Haji picked up a dead leg, he had a difficult time in the game.
‘However, with that injury, I don’t see that as a problem going forward.’
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