Richmond are at a loss to explain why Ty Vickery had to come off during last week's AFL pre-season match against Fremantle.
The Tigers have ruled out heat stroke and are diagnosing him with concussion.
He will not play in Saturday's NAB Challenge match against Hawthorn.
Vickery became confused during the game and thought he was in a marking contest, despite being well away from the ball.
Opponent Alex Silvagni alerted an umpire to Vickery's state and he soon left the field.
Vickery did not report any head knock and the Tigers told the Herald Sun on Wednesday there is no match footage of the ruck-forward suffering high contact before the incident.
Fellow ruckman Ben Griffiths will also have to pass a concussion test after opponent Aaron Sandilands caught him high during the match.
Sandilands has accepted a one-game rough conduct ban for the incident.
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