According to Kenny Jackett, the 29-year-old made the admission when weighing up the necessity of an operation.
The problem has dogged Brown this term, with the defender missing six weeks earlier in the campaign in the hope rest would alleviate the issue.
Despite a mid-October return, coinciding with an uplift in the Blues’ form, the ex-Bristol Rovers man went under the surgeon’s knife earlier this week.
And Jackett anticipates the defender’s return in early February.
Pompey’s boss said: ‘Lee’s never really shaken it off and, in his own words, always felt during the course of this season that he hasn’t really been at 100 per cent or played at his best.
‘During the Peterborough game, he felt it was too much and couldn’t wait until the end of the season or try to play through it.
‘So an operation after seeing a specialist was probably inevitable. Get it done as soon as possible, with a good chunk of the season left.
‘Operations are always the last resort with any injury, but different things had been tried, such as injections and periods of rests when he missed games.
‘Lee has been a very consistent player for me in his two seasons here and I always listen to players, people know their bodies.
‘He feels it has been getting increasingly worse and not better – and because of that has hindered his performances and will only get worse.’
With potential left-back deputy Brandon Haunstrup sidelined by a knee injury, Jackett is struggling to fill the role.
Anton Walkes is expected to retain the position against Ipswich on Saturday, having stood in several times during his Blues career.
Jackett added: ‘An Achilles is a nagging niggling injury which can stop you turning or hurts in the first couple of yards off the mark.
‘As soon as Lee got over the opening 5-10 yards, he could run no problem, but around our box, twisting and turning was causing him some discomfort and pain.
‘It was hindering his mobility, that is the player’s words.’