Bailey is the only change to the starting line-up from the one that got off to a winning start against Bath last weekend with a 24-19 victory at Kingsholm.
England centre Mike Tindall has not fully recovered from a calf strain and is not included, while Jack Adams, who grabbed the first try last Saturday, is named on the bench.
"Iain's injury is not too serious and there is no structural damage to his ankle but unfortunately he will miss out this weekend and so will Mike Tindall," head coach Dean Ryan told www.gloucesterrugby.co.uk
"This can be a difficult time of the season if you don't manage to get a win in the first couple of games because it increases the pressure. We know everyone will lose matches in the next nine months but we are in the nice position of not looking around at where our first win will come from.
"But we know Harlequins will be a very competitive side and a difficult ask for us. They played their first game at Twickenham last weekend which is tough, but they have players who are used to the big stage and have also made some high quality signings who are used to the intensity of the Premiership.
"It will be a big day for them at The Stoop but we have great ownership in this side and know exactly what we want to try and achieve and at present I am really pleased where we are."
In the front row, Nick Wood is named at loosehead, Mefin Davies at hooker and Christian Califano at tighthead.
Marco Bortolami is named as matchday captain in the second row alongside Alex Brown. The back row of Peter Buxton, Andy Hazell and James Forrester also remains unchanged.
Peter Richards partners Willie Walker at half-back, while the Anthony Allen and Rudi Keil centre partnership is also unchanged.
Bailey, Mark Foster and Olly Morgan complete the back three.
GLOUCESTER (v Harlequins, tomorrow, 3pm): Olly Morgan; Mark Foster, Rudi Keil, Anthony Allen, James Bailey; Willie Walker, Peter Richards; Nick Wood, Mefin Davies, Christian Califano, Marco Bortolami (capt), Alex Brown, Peter Buxton, Andy Hazell, James Forrester. Reps: Olivier Azam, Jack Forster, Will James, Luke Narraway, Rory Lawson, Ludovic Mercier, Jack Adams.
