A LEANER and harder-working Billy Longer says he is ready to deliver as St Kilda's No. 1 ruckman.

Longer, who turned 24 on Friday, returned to the Saints' senior side in round five against Geelong for his first AFL game since 2015.

Longer replaced the out-of-form Tom Hickey, who suffered a medial strain in his left knee last week and won't be available until after the bye.

After being selected with the eighth pick in 2011 by the Brisbane Lions and having played just 46 matches, Longer knows it's now time to deliver.

"I put that on myself. Even players have said 'if you had an opportunity before, you definitely have one now'," Longer told AFL.com.au.

A trade at the end of 2013 saw the Saints give up No. 25 and 41, picks which turned out to be Daniel McStay and Jake Kolodjashnij (Geelong) respectively. They also received No. 48, which was on-traded to Greater Western Sydney for Josh Bruce.

The club announced last July that Longer had re-signed until the end of 2018, but not before asking what the ruckman what he wanted to do.

"I said pretty firmly I was pretty comfortable and happy with St Kilda," he said.

St Kilda was happy to oblige, having seen what Longer could do when fit.

The big man held the ruck mantle in 2015 and played 18 matches, but had his season ended early by a right shoulder injury that eventually required a reconstruction.

"It was sore for a long time (after the operation) and to be honest, I wasn't really sure if it had healed properly," he said.

"I was getting a lot of feedback that I was going slower than I should be, but it wasn't until probably three-plus months after the surgery that I ended up getting a cortisone mix with something else, which made it feel almost brand-new again.

"For the first time in three-plus months of rehab, I was finally able to get it to move properly. From then on, strength training was so much easier."

Longer overcame that problem, but delayed concussion plagued him. Standard head knocks affected him more than normal – he suffered lingering headaches after games and had difficulty holding a conversation.

"After each successive head knock, the recovery got worse and worse, to the point where I knew that something was up," he said.

"I saw some specialists and everything across the board, like my memory and balance and reaction time and all that sort of stuff were pretty far down, which isn't ideal at all. It was something I took pretty seriously."

Despite his difficulties, the process made him hungrier. St Kilda ruck coach Adam Skrobalak said Longer's shift in attitude was noticeable.

"He's working a lot harder around the club and on the training track," Skrobalak told AFL.com.au.

Becoming a better runner was vital for Longer to adapt the third man up no longer being allowed.

"You can't have your mids chopping you out and coming over the top – you have to compete in every stoppage," Skrobalak said.