DEVELOPING Kangaroos key defender Declan Watson faces a year out of football after rupturing the anterior cruciate ligament in his left knee at training on Thursday.

Watson, 18, crumbled to the Arden St surface clutching his knee early in the session and was quickly surrounded by North Melbourne trainers.

He was able to walk from the ground with a trainer on each side of him, but was visibly distressed. The Roos' medical staff immediately sent him for scans that confirmed their worst suspicions.

"There's the two types of ACL – the contact and the non-contact. This was a non-contact ACL," coach Brad Scott told radio station 3AW.

"It was a really innocuous, slight change of direction and his knee buckled and we all feared the worst – and that's been confirmed, unfortunately.

"So it's an ACL rupture and that will be 12 months (out) for Declan after his best game at North Melbourne (in the VFL) last week, so it's bitterly disappointing for him and for the club."

Watson returned in the VFL for Werribee at the weekend after a long absence with ankle and shin problems.

The 194cm Aspley product was slated to resume in the Development League, but a player shortage saw him play in the seniors and he was a strong performer. Watson spent time on Hawthorn-listed Ty Vickery, who kicked just one goal.

Watson ends his first season on the Kangaroos' list with seven VFL appearances, including three at senior level. He was the Kangaroos' second pick in last year's NAB AFL Draft at No.34 overall.

Declan Watson leaves the track at Arden Street after injuring his knee. Picture: AFL Photos