The Australian, sent off for a second booking for kicking the ball away in injury time, will miss the home match with fellow relegation battlers Portsmouth on Sunday April 25.
"It's a blow to us Mark being sent off, especially as he misses the Portsmouth game," said Gray.
"Sometimes these things happen on the spur of the moment, but it was disappointing, and Mark is disappointed himself because he knows he should not have done it.
"To lose a player of his ability is costly and we will have to cope without him when he is suspended, but Alan can lead the line well."