Shh, don't tell Armstrong. If he thinks he's in the under 21's and sees his name isn't down he might not turn up for the game and will therefore be able to play for us.
There is a re-call option but his manager Steve McClaren has said it will he Armstrong's choice whether he stays here or tries his hand at a higher level, the Championship or obviously goes back to the Toon
He's under 21 and been there longer than 3 years so doesn't need to be named in their squad to be able to play for them. (Football Manager knowledge coming in handy here).