WEST Coast milestone man Brad Sheppard used to doubt whether he belonged at AFL level, but feels indebted to coach Adam Simpson for igniting his career after taking over at the Eagles at the end of 2013.

A perennially underrated defender, Sheppard will reach the 150-game mark and qualify for life membership in Sunday's 48th Western Derby.

The 27-year-old has become one of the most reliable and consistent small backmen in the competition, regularly going head-to-head with stars like Robbie Gray and Michael Walters with a minimum of fuss.

But that wasn't always the case, with Sheppard admitting his struggle to adapt to the professional lifestyle and his eagerness to make it AFL level hindered him early after he was drafted via pick No.7 in 2009.

"I was quite impatient the first part of my career. The backline we had at that stage, we had a really experienced group," Sheppard said.

"I was so eager just to play at the top level that I put my hand up to go forward and it didn’t suit me ideally, because I feel like I play my best footy behind the football.

"But, in saying that, I think the lessons I learned playing forward have made me the player I am today.

"'Simmo' came in and said, 'I want you to play in the backline, I think that's where your strengths are and where you are going to be the greatest asset for this side'.

"He plonked me on the half-back line and since then I haven't moved out."

Despite often copping a battering, Sheppard has strung together 90 consecutive appearances and been a bankable big-game performer – named West Coast's player of the 2015 finals series and among the Eagles' best in their shocking Grand Final loss to Hawthorn.

"I feel like I've got a good balance at the moment between my attacking game and my defensive game," he said.

"Playing against these strong forwards, you need years in the gym. I know I don't look like the biggest bloke on the field … but just to be able to have that strength to be able to compete.

"Because in big finals and big games you need to be able to defend one-on-one.

"I think that's a real strength of mine, to be able to win or halve contests."

Courageous to a fault, Sheppard is regularly one of the last Eagles onto the training track early in the week.

But he has found a regime that works for him and become a permanent fixture in a strong defence alongside skipper Shannon Hurn and Jeremy McGovern.

"You think, being an early draft selection, things are going to happen straight away and when they don't you doubt yourself," Sheppard said.

"But I've got real confidence in my body. 

"It hasn't been an easy journey for myself, fighting a bit of adversity early in my career – whether it be form or injury.

"To be able to string together enough games to be a life member is something I'm really proud of."