Heffernan was sent off after aiming a headbutt at Shrimpers defender Charlie Mulgrew with three minutes of the Roots Hall clash remaining, and will now miss next Friday´s second leg at the Keepmoat Stadium and a potential trip to Wembley.
"I didn´t see it but I spoke to the referee and he said it was a headbutt and we have to accept that," said manager Sean O´Driscoll.
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