The then 18-year-old had only recently arrived at Elland Road from Morecambe's reserves and found himself up against significantly more experienced Aston Villa players such as Jonathan Kodjia.
Speaking to BBC Radio Leeds on Wednesday, Arnott joked that Davis was 'nearly sent off after 30 seconds'.
But as Arnott explained afterwards, the Geordie native recovered to pass his first test.
"He was up against Jonathan Kodjia, one-versus-one, and Kodjia basically just pinned him, grabbed him and wouldn't let him move," he recalled. "He could receive it to feet and had pretty much most of the control of the ball at that period. To be honest, Leif dealt with it really, really well.
"I'm sat there watching thinking we as a staff maybe haven't prepared him well enough, because he was, in my opinion, struggling to win the ball back, although he wasn't doing a bad job. I was thinking how can we help him, how can we improve him? Maybe that's an area of his development we've missed.
"I must have pondered over it for 48 hours, speaking to multiple people, watching different videos, really gone to town and thinking I'd done a bit of a disservice to Leif. We spoke about it as a staff and it ended up coming up in conversation with Liam Cooper on the training ground, just as a quick one saying, 'we've noticed this about Leif, what would you suggest?' Because he deals with it relatively well when he's pinned, obviously he's fantastic in that one-v-one situation. He just looked us in the eye and said, 'it's Kodjia - he pins everybody in the Championship. He'd do that to me'. Then the penny dropped a little bit in our heads.
"At times as a staff you're so focused on trying to help the player, trying to develop the player that sometimes you forget about the challenges you're up again and also the realism of that challenge. Kodjia is a seasoned pro, experienced, saw Leif had just come on to the pitch, probably saw an opportunity to go and test him and ultimately Leif did really, really well.
"I think since then his confidence has improved and his ability to deal with those situations has improved."
Davis has gone on to play nine more times for Leeds' first team, and was rewarded with a new three-year contract on Tuesday,
Kodjia is, of course, now a Premier League player after Aston Villa's promotion last season.
Leeds fans - what do you remember about Davis's debut at Aston Villa?
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