Pham is asymptomatic and self-isolating, Preller said, adding that he has been in contact with Pham every day.
Preller said the team is following contact tracing protocols and testing protocols. Pham will need to test negative twice in a 24-hour period before returning to the field amid the coronavirus pandemic.
"When Tom clears all the boxes, we'll talk about him returning to play," Preller said.
Teams may release the names of players who test positive only when the player gives his authorization.
"I think what we're going to learn is this is going to be a very fluid process," Preller said at Petco Park, where the team was working out with a full squad for the first time in the summer training camp. "You might have a full camp one day and not the next, because there is so much testing going on. ... It is literally going to be like every day."
Pham, 32, joined the Padres in the offseason, coming over from the Tampa Bay Rays in a December trade that also netted the Padres prospect Jake Cronenworth in exchange for outfielder Hunter Renfroe. Pham hit .273 with 21 home runs and 68 RBIs for the Rays last season.
The Associated Press contributed to this report.