Sri Lanka's captain Dinesh Chandimal warned meanwhile that conditions were "very different" to England, with some "mystery spinners" who could pose problems for Eoin Morgan's batsmen.
"Very happy being favourites. It shows we've been doing good stuff and I like being favourites," Buttler told reporters, saying the team were "very confident" and aiming to be "pushing the boundaries".
The batsman added however: "We're very aware of Sri Lanka's qualities in these conditions. They have some experienced players... It's been tricky in the past for England sides to come here."
He added that being the number-one ranked one-day side ahead of next year's World Cup on home soil brought its own pressures.
"There is always pressure being number one. Once you get there, I think that's when the hard work really starts. You've got a bit of a target on your back.
"It is down to you to keep improving and try to stay there. That's the challenge and it has its own pressures. I think in the dressing room we have a good understanding... and know that we need to keep improving and pushing the boundaries."
Buttler said the World Cup was in the back of everyone's mind, but they were taking every series one at a time and hoped to build a core group of players for next year's championship.
"It's about trending in the right way and being able to build a core group of players -- a squad of 15, 16, 17 guys -- who we can call upon hopefully for the World Cup," he said.
Underdogs Sri Lanka are still reeling from the axing of captain Angelo Mathews after a dismal run of 30 losses in their last 40 ODIs including defeats to Bangladesh and Afghanistan in last month's Asia Cup.
Chandimal, who missed the Asia Cup and the preceding ODI series against South Africa due to injury and suspension, cautioned that home pitches had "a lot of spin".
"Especially for the batters it's not easy to play... We have some mystery spinners as well.
"They got all the experience and talent and we have to compete with them. When the situation comes we have some game plans which we will have to execute."
Chandimal said he hoped to improve the team's fielding.
"In the last few days, we worked really hard on our fielding individually and as a group," he said.
"That's the biggest change we are trying to make in this series -- give priority to our fielding and give it a lot of attention."
The five-match one-day series is followed by three Tests in November.