The 32-year-old keeper - who had 11 successful years with Glentoran and four with Crusaders before coming to Shamrock Park - was stabbed in the left arm and the left side of his chest near his Lisburn home after he protected his sister.
He told the Portadown Times he was returning home in the early hours of Sunday morning with his sister when a number of men emerged from a house party and verbally abused her.
He explained: "I told her to walk on and exchanged words with the men when one of them produced a knife and stabbed me. I just happened to be in the wrong place at the wrong time. It was very frightening and very painful."
He added: "I was rushed to hospital where I spent three nights and underwent surgery for both wounds. I am still in pain and cannot lift my left arm - it's still extremely painful and I won't know until two or three weeks time what the prospects are. I can't look to the future, and I suppose at 32 my career would almost be over anyway.
season
"But I like being part of the scene at Portadown and would like to think I have a season or two left in me. Goalkeepers usually last longer than outfield players."
The stitches were removed from Armstrong's wounds on Monday, but he won't know for a few weeks yet how the wounds will affect him long term. "It's all according to how well the muscles in my arm heal and after that I'll undergo physiotherapy," he said. "It's been a frightening experience and my main emotion is one of anger that anyone could attack another human being with a knife."
Armstrong, who signed for Portadown last season as an understudy to first choice David Miskelly, was at Shamrock Park on Saturday to watch the Ports' narrow 2-1 CIS Insurance Cup win over Larne - he was taken there by his friend and club mate David McCullough.
"I'm not able to drive yet," he said. "It as a good day out and just what I needed, and I hope I can come back from this."
Portadown manager Ronnie McFall said that the players and officials at Shamrock Park were deeply shocked at the incident and added: "Naturally we hope Neil make a full recovery. He's a true professional and nobody deserves the likes of this. We wish him all the best."
attacker
Lisburn police are still searching for the attacker and ask anyone with information to ring the local number 0845-600-8000 or Crimestoppers on 0800 555111.