The 18-year-old has scored 13 league goals for the Sky Blues since his move to the club in July.
Armstrong's streak of goals has helped propel Coventry to second spot in League One, prompting the following praise from his boss.
Mowbray told this week's FL72 Podcast: "I'd been managing in Middlesbrough so obviously had been watching Newcastle U21s a fair bit, and knew about him.
"What I found is that the first day he came onto our training pitch you could see the power, the fluency, the touch, it was just a bit beyond a lot of the rest the group.
"When you put it into context, he's an 18-year-old boy, we thought: 'Wow, we've got one here.'
"Thankfully for us, and him, and hopefully in the long run for Newcastle it has been a good thing for him."
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