Speaking in an interview on Tesco’s YouTube channel, Lewis, who was parachuted in a month earlier than originally announced after 27 years with Unilever, said it was "pretty clear" the retailer needed to "get back to the core of our business".
"We have to get back to a place where we have offered to our customers an experience that is better that anywhere else," he said.
"Now what that needs to be - to be better - we still need to work that out together. But we will go back to the core; we will go back to finding out what it is.
"Be it price, be it service, be it availability – we need to work out what it is that customers need from us so they will reward us with their loyalty."
Admitting the retail giant, which issued a profit warning earlier this month, had struggled with reputation, he said: "I’m not naive to the position of the brand and the reputation but I only see that as an opportunity.
"Look at it another way – the great thing with brands is it’s easy to talk, but actually you change reputation by doing. So many customers come to us on a daily, weekly basis – if we can change how it is they experience us then the opportunity for turning around that reputation is massive."
Earlier this week, Lewis wrote a letter to staff, promising the retailer could regain its position as "customers' champion". His comments again reveal a focus on turning around customer service.
"In our DNA we have always been the customers' champion, and we will be again," he wrote.
Lewis joined Unilever in 1987 and held a number of marketing roles, including marketing operations manager and marketing director for Unilever’s River Plate business in Argentina, Uruguay and Paraguay.
He eventually became Unilever's president, personal care, before joining Tesco this week.