He first dislocated his shoulder in a Saints reserve match against Portsmouth in December 2005 following a crunching challenge from Linvoy Primus.
He said: "It started to pop out quite frequently in the second half of last season and soon it became an absolute nightmare.
"We had to keep quiet because if the opposition heard about it, the first thing that happens is someone will shoulder-charge you."
Walcott then fell heavily in a game against Everton - the day after his 18th birthday last March - and there was no hiding the seriousness of the injury.
Tests revealed his body had already been put under too much strain and the condition would need an operation to tighten and repair the ligaments.
Surgery was completed within days and Walcott added: "Everyone could see the pain I was in. It had got to the point where even just drying myself with a towel or rolling over in bed, the shoulder would pop out.
"When I woke up after the surgery I was absolutely in bits. My arm was completely floppy after the anaesthetic.
"But it's all over now. They say I won't be able to get full extension but it's 95 per cent. I just work and train hard, that's all you can do really."