Manager Bud Black said Richard has lost 12 pounds since getting sick during a series at San Francisco two weekends ago, although he's started to gain it back.
The move came a day after Richard fell to 0-4 with an 8.54 ERA after an 8-1 loss to Arizona on Saturday night, when he was knocked out in the fourth inning.
"We knew this was been going on. But it finally got to the point where, hey, let's not beat our heads against the wall here," said Black, who met with Richard before Sunday's game and told him of the move.
"He's sick," Black said. "He's getting a little better, then he sort of digresses, then he gets a little better. It's more than just how you get a virus, you throw up, you have diarrhea, two or three days later, you're a little weak, then you start gaining on it."
Richard's scheduled start at San Francisco on April 20 was pushed back three days. The lefty then had the first of three straight bad starts, lasting only 1 2/3 innings in a 6-3 home loss to Milwaukee.
Black said he thinks the illness has affected Richard's pitching.
"For me, yeah. I mean, he'll tell you no, but I think it has. Again, when you've lost weight, you've lost some strength, that's had some bearing on his performance."
With two off days in the next two weeks, the Padres will go with a four-man rotation.
Ross, who has been on the DL with a shoulder injury, will pitch out of the bullpen. He's made three starts for the Padres this year, going 0-1 with a 3.86 ERA. He made two rehab appearances for Triple-A Tucson. He was acquired in an offseason trade with Oakland.