The left-back was forced off with an Achilles injury in the first half of last week’s 3-3 draw against Coventry.
Initial fears centred on the Achilles being ruptured, potentially ruling him out of action for the campaign.
However, once Pompey’s medical team were able to fully assess the damage, the prognosis is an absence of between 4-6 weeks.
That opens the door for the ever-patient Brandon Haunstrup to be handed a run in the side.
Intriguingly, former Millwall full-back James Meredith has been training with the Blues since the tail-end of this week.
It remains to be seen whether the free agent’s south-coast presence will develop into a more long-term arrangement.
Aged 31, he made 36 appearances in the Championship last season, before leaving The Den in the summer after his contract was terminated.
If that situation does develop, it would be harsh on Haunstrup, who is now in his second season serving as Brown’s left-back understudy.
The 22-year-old claimed an assist for John Marquis’ goal after appearing from the bench against the Sky Blues.
He made 12 appearances last season, of which two starts came in League One as Pompey finished fourth.
Overall, Haunstrup has made 40 outings since handed his Blues debut in August 2015 against Derby in the Capital One Cup.
The Waterlooville youngster has so far made two appearances this season, both from the bench.
Meanwhile, Brown had been an ever-present during Pompey’s opening five matches of the current campaign.