The Tottenham Hotspur midfielder replaced Armstrong midway through the first-half of the 2-0 semi-final victory on Sunday after the Newcastle United striker received a knock on his ankle.
Armstrong was clearly upset as he left the field, but the tears of pain spurred the Young Lions on to claim a place in the European Championship final according to Onomah.
England v Holland
UEFA European U17 Championship
6pm, Wednesday 21 May 2014
National Stadium, Malta
Live on Eurosport
"It wasn't the best circumstances to come on," he told TheFA.com. "I was gutted for Adam when he got his ankle injury, and he was a bit upset - and I think that inspired a few of the boys to really push on and get the win.
"We have all said that if he doesn't make the final then we are going to win it for him.
"We know Holland are a good side, but if we stick together we can do it."
The saying 'walking into the lion's den' could not be more fitting to the situation that faced Onomah at the Ta'Qali National Stadium. He watched on as a combination of the crossbar and post came to England's rescue several times during the first-half - but a determination to impress the manager saw him produce a man of the match performance.
"I'm buzzing. It was our target before the tournament to reach the final and now that we've done it I can't wait for that whistle to blow. We are very confident, we've got a good team, everyone is united and we believe that we can do it.
"I was sitting on the bench and they hit the woodwork three times, and I feared the worst," admitted Onomah. "When I came on I knew I had a job to do and stick to my task.
"It was very difficult coming on in that situation. The tempo is very quick and you have to get involved straight away."
He continued: "I think it was my best performance for England. I was disappointed not to be selected in the starting XI, so I wanted to impress when I came on and I could have scored one or two as well.
"I was so glad to win the game, and so happy for Dom [Dominic Solanke] and Pat [Patrick Roberts] when they scored. It will give them a big boost."
And it was following Solanke's opener that Onomah showed the world his moves - joining the goalscorer, Tafari Moore and Joe Gomez for a celebratory dance.
Onomah explained: "We had been practicing it all week, and we were confident one of us was going to score. Taf wanted us to do a little dance, as he's got some good moves, so when Dom finished it we just did it, and I thought we were quite good.
"Izzy [Brown] was supposed to be involved too - but he bottled it!"
Now with Wednesday's showdown with Holland to look forward too, Onomah says the last few weeks have been "brilliant", but he's looking to end it in style.
"This is the best experience of my career so far and not many people can say they've been involved in a European Championship Final - and in a few years I'm sure I'll be able to look back on this and be very proud.
"We all know how we want to end the tournament though, we want to win it."