SIDELINED St Kilda midfielder Koby Stevens is struggling to train as he battles concussion, with his indefinite absence from the field continuing.

Stevens suffered a head knock against Brisbane in round one when he limited star Lion Dayne Zorko to 14 disposals. He played the game out and spent one more week in the senior side before being dropped.

An emergency for round three, he did not play at any level and reported concussion-related symptoms leading up to the following week's Geelong clash.

He was diagnosed with delayed onset concussion and given a few days away from the club.

Almost a month later, there is no timeline on Stevens' return, and St Kilda's football manager Simon Lethlean told Melbourne radio station SEN the tough onballer's situation was comparable with defender Dylan Roberton, who has been ruled out for the season after experiencing an irregular heartbeat and collapsing on-field against the Cats in round four.

"With Koby, it's a bit like Dylan in some respects. He's on a modified exercise program, such that you're only allowed to get your heart rate to a certain level and if he increases it and feels unwell, he brings it back down," Lethlean said.

"He was on the bike today (Thursday) and couldn't get past a certain level because he didn't feel too well.

"He's not particularly well at the moment, insofar as how he's feeling. It's certainly affected his concentration and a few headaches, and some other things.

"He's being treated with a lot of caution but at the same time, with the intent that he works his way into coming back. Just not sure when it's going to be."

Lethlean played down the notion the concussion could be career-threatening to the 26-year-old.

"The intent is that as soon as Koby feels well enough as he exercises more, he'll be back into it," Lethlean said.

"I guess there's a degree of uncertainty with any of these areas that you're heading into waters where we're all learning the effects on athletes of head knocks.

"I don't think there's any certainty around it, but there's certainly no dire prognosis at the moment either."

Meanwhile, Roberton who made the Virgin Australia All Australian squad of 40 last season, has had a battery-sized device implanted into his chest so his heart can be monitored for the next three months.

"I think it was an irregularity of the heartbeat," Lethlean said.

"We got an explanation from the doctor that's far more technical than that, about various areas of the heart that are in charge of sparking the right pumping mechanisms."

Similar to Stevens, Roberton is limited in how much exercise he can do.

"He's feeling well, so it's all a bit foreign to him," Lethlean said.

"He's been feeling well ever since, but the tests have shown that they can't quite pinpoint why what happened, happened, and why there's an irregularity there.

"Until they can be comfortable that's not going to occur again, he's not to get his exertion and stress levels to a degree that could re-spark that issue."