The Ulster centre suffered another head injury during last Saturday's opening Test against the Pumas – although it has not been diagnosed as a concussion.
The IRFU say that their medical staff are taking extra precautions with Marshall this week in the wake of the series of concussions that he suffered last season – but he hasn't yet been ruled-out of playing this weekend.
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Conor Murray is the other main fitness concern for head coach Joe Schmidt because of a dead-leg but his half-back partner Johnny Sexton is expected to be available after shaking-off a cramp to his knee.
