Mr Newsom was blasted by Representative Barbara Lee for his comments to Chuck Todd on NBC’s Meet the Press.
The health of 90-year-old Ms Feinstein, a Democrat from California, has come under close scrutiny after she missed three months of work while hospitalised for shingles.
She also suffered a fall at home last month but has rejected calls to resign while saying she would not run again in 2024.
Mr Newsom has previously promised to nominate a Black woman to replace Ms Feinstein if her seat in the US Senate ever became vacant.
Speaking on Meet the Press, Mr Newsom was asked by Todd if he would stand by his decision and if “it would be essentially a caretaker, an African-American woman?”
“Uh, we hope we never have to make this decision, but I abide by what I’ve said very publicly and on a consistent basis, yes,” Mr Newsom responded.
Ms Lee, who is a Black Democrat from California running to replace Ms Feinstein, called Mr Newsom’s comments “insulting.”
“I am troubled by the Governor’s remarks. The idea that a Black woman should be appointed only as a caretaker to simply check a box is insulting to countless Black women across this country who have carried the Democratic Party to victory election after election,” she said.
“There are currently no Black women serving in the Senate. Since 1789, there have only been two Black woman Senators, who have served a total of 10 years.”
Anthony York, an adviser for Mr Newsom, told KTVU that the governor had responded to “a hypothetical on top of a hypothetical.”
“There is no vacancy for any US Senate seat, nor does the Governor anticipate there will be one. Voters will have their say on who should replace Senator Feinstein when they go to the polls less than six months from now.”
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies