Thomas' wife is expecting their baby and has gone into hospital. Andrew Davies is on standby to take his place.
The Welsh side are 14.5 points behind third-placed Essex and victory is vital to keep them in the hunt for promotion.
Coach John Derrick said: "It looks a good wicket and I think it will come down to spin at the end so we've named two spinners in the side.
"It's a must-win game for us to keep us in contention for promotion."
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