Watson is recovering from a hamstring injury suffered in last Saturday's one-day international loss to India in Bangalore.
The 32-year-old will not play for New South Wales in next week's Sheffield Shield clash against Queensland in Brisbane in order to give him the best chance to line up in the Ashes opener.
Watson's absence from the Blues' team means he will not play a lead-up four-day match before the first Test.
"The reports from our medical team on Shane's recovery from injury are pleasing," Lehmann said in a statement.
"He is making good progress and responding well to treatment. If that continues, we have confidence that he will be fit for the first Test.
"We won't be taking any chances with his preparation though, and he will miss next week's Shield match between New South Wales and Queensland at Allan Border Field.
"We want him to continue his treatment, and if all goes to plan, join the Australian team in camp ahead of first Test in Brisbane."