Phelps, the 18 Olympic gold medallist, returned to competition in April after a retirement that lasted nearly two years and finished this month's US Championships without a title.
Phelps, 29, is preparing for his first international meet since the 2012 London Olympics after ending his retirement from swimming.
The greatest Olympian of all time will contest the 200 metres individual medley and the 100 metres butterfly at the four-day PanPacs starting on Thursday as part of a 60-strong US team also featuring Ryan Lochte, Missy Franklin and Katie Ledecky.
"But I am taking it a lot slower now than what I did before because I do understand that I need to give my body time to get back and build up to where I want to be.
"I can't just expect to do everything I want from the get-go. Being able to have a year-and-a-half off, I've been able to learn a lot about myself and I still have that competitive side.
Phelps, who has won 22 Olympic medals, sealed his place in the US team for the PanPacs with a second silver medal at the US national titles last week, finishing just 0.05sec behind world record-holder Lochte in the 200m individual medley.
Lochte said he knew Phelps would return to the sport, despite quitting for a time after the London Olympics.
"I think I was the first one to say that he's going to come back," Lochte told reporters. "Once you have that competitive edge in you, it's hard to get rid of. He missed being in that water, getting up on the blocks and the excitement of racing the top guys in the world."