South Africa international Watson is rested after suffering a knock during the victory over Harlequins last weekend, while former England prop Flatman could be sidelined for up to four weeks with a torn bicep muscle.
Bath skills coach Brad Davis said: “Luke has played a lot of rugby and, coming off a Super 14 season, there is a bit of wear and tear. We wanted to give him a break and freshen him up.
“You want your best players available, but you’ve also got to use the squad, otherwise you end up running them into the ground.”
Bath have taken 30 from a possible 35 points in a surge towards the Premiership play-off places, but Leicester are unbeaten at home in 29 games stretching back to September 2008.