The Newcastle United boss feels the 17-year-old initially struggled against Crystal Palace.
The 17-year-old was making his full debut for the Magpies during the Capital One Cup tie earlier this week, which Pardew's side won 3-2 after extra time.
However, Armstrong had attended his grandfather's funeral just 48 hours previously - and it looks as though it affected his performance initially, with the Newcastle boss admitting he looked nervous.
Armstrong later dedicated his performance to his late grandfather, while Pardew says the youngster was 'terrific' after one bit of magic changed his game.
“The first five minutes I would give him zero out of 10, he was a bit nervous," the 53-year-old told reporters.
“Then he had one little bit of magic that gave him the confidence. I thought he was terrific after that."
Newcastle's 3-2 triumph was their first of the season - but they are still yet to find a win in the Premier League.
They travel to Stoke City on Monday night in hope of banishing that statistic, with a result much needed in terms of lifting them off bottom spot in the table.