Alese linked up with the Championship side at the end of the January window, with the defender desperate to get some much-needed minutes under his belt and get back on track after a string of injury setbacks at Sunderland.
But the centre-half played just four minutes for Pompey before suffering a quad injury that looks set to sideline him for four months.
Portsmouth boss John Mousinho is gutted for Alese but has admitted Portsmouth are now looking into the possibility of de-registering the 25-year-old from their squad for the remainder of the Championship season.
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Alese's last Sunderland appearance came last January when he suffered a broken leg in the FA Cup tie against Stoke that forced him to miss the Championship run-in.
He then suffered a further setback that meant he missed the early months of the Premier League season, and headed for Portsmouth determined to get his career back on track.
But he's now set for another lengthy spell out of action.
Portsmouth boss Mousinho said: "Aji is obviously disappointed, he felt he had settled in really well. It’s a tough one when you pick up that many injuries, he’d been out for that long and had a couple of different injuries.
"He felt he was in a good space. Aji had come back (from injury) and played 45 minutes for Sunderland, then 60 minutes for Sunderland, then built up in training with us for what we thought was a sensible amount of time to play.
"Unfortunately it doesn't always work out in a straightforward manner but he’s a really good kid and I am sure he will come back stronger."
Mousinho added: "Most likely his season is over. He’s back at Sunderland, if we can get him back quicker then great, we’ll see what that looks like. He’s obviously still our player - but we are preparing for it to most likely be over.
"Aji was doing pretty well, I thought he had settled in very well, he was positive when he came in against Preston. It’s impossible to tell how much of an impact he was going to have based on that, but we were pleased.
"It happened after about five minutes of the game. We had made five subs anyway, although I’m sure it wouldn't have made an ounce of difference with the injury.
"We can apply to deregister Aji, it depends on the actual length of the injury, verified by an independent consultant - but we are going to try.
"We deregistered Ibane [Bowat] last season with an injury he picked up in September 2024 and it was a clear nine-ten month injury, so it was really easy to justify at the time that he wasn’t going to be able to play before the January.
"If you pick up an injury now, you have to make sure there’s a clear four-month injury, so it’s going to be 16 weeks before the player returns. It is slightly different in terms of what you have to demonstrate.
"We don’t need Aji’s space at the moment, we’ve still got two free spots. It would be useful to have, but we don’t need it."