International call-ups have deprived head coach Dean Ryan of both England's Peter Richards and Scotland's Rory Lawson which means Thomas will make his 16th Premiership appearance and second this season after he came off the bench against London Irish a fortnight ago.
Ryan has also been forced into two other changes for tomorrow's trip to the Memorial Stadium. Willie Walker will start at number 10 because Ryan Lamb is ruled out with damaged ribs and that means Olly Morgan returns to the full-back position.
The rest of the starting XV is identical to the one that beat Wasps 27-21 on Saturday to continue Gloucester's unbeaten start to the season.
"We have obviously lost players to international call-ups, but probably not the ones we were expecting at this stage," Ryan told www.gloucesterrugby.co.uk "That has given Haydn a great opportunity to show everyone what he can do.
"We need him to fit into the team structure, do the basics in his game very well and the rest will follow on for him."
Ryan has also recalled Brad Davies from his loan at Pertemps Bees and he will sit on the bench and cover both half-back positions because Ludovic Mercier is ruled out with a neck problem.
Gloucester have an excellent recent record against Bristol and have lost only one of their last 11 meetings - 20-15 at Kingsholm in March.
But Ryan knows they will be in for a tough examination of their unbeaten start to the season tomorrow.
"I think once you get seven or eights games into the season then form tends not to lie and Bristol are challenging at the top of the table totally on merit," he said. "They have a fantastically settled and experienced side and have beaten some very good teams already this season.
"I am sure it will be a passionate, intense occasion and we will have to be at our very best."
Gloucester's front row remains intact which means Nick Wood starts in the number one shirt, Olivier Azam at hooker and Christian Califano at tight-head.
Adam Eustace will partner Alex Brown at lock, while Peter Buxton, Jake Boer and Luke Narraway will form the back row.
Thomas and Walker will start together for the first time at half-back; Mike Tindall will partner Anthony Allen in the centre, while Morgan will complete the back three alongside James Bailey and James Simpson-Daniel.
On the bench, Gloucester have opted for a five-two forward split and include a complete front row. Hooker Rob Elloway and props Jack Forster and Patrice Collazo will be joined by lock Jonathan Pendlebury, number eight Adam Balding, centre Jack Adams and utility back Davies.
GLOUCESTER (v Bristol, tomorrow, 7.45pm): Olly Morgan; James Bailey, Mike Tindall, Anthony Allen, James Simpson-Daniel; Willie Walker, Haydn Thomas; Nick Wood, Olivier Azam, Christian Califano, Adam Eustace, Alex Brown, Peter Buxton (capt), Jake Boer, Luke Narraway. Reps: Rob Elloway, Jack Forster, Patrice Collazo, Adam Balding, Jack Adams, Jonathan Pendlebury, Brad Davies.
The Shed bar will be open at 6pm tomorrow to show the Gloucester match and again on Saturday for England's clash against South African.
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