Richard, who is 0-3 with a 13.09 ERA in his last three starts, has struggled since initially his scheduled start on April 20 was pushed back because of a virus that caused him to sit out that day in San Francisco.
"It was really bad in San Francisco, then I started to feel better and then it went the other way again," Richard said after Sunday's game. "Now, it's being able to take a step back to get a better understanding of what's going on."
Richard said he had a similar experience five years ago when he was in the Minor Leagues with the White Sox, where he was sick and eventually had a relapse. But in that case, the virus simply went away.
"Initially, we thought it would be a few days," said Padres manager Bud Black, who said that Richard has lost 12 pounds in two weeks. "It's just hung on and he just can't shake it."
Richard will see a gastroenterologist on Monday. He allowed five earned runs in 3 2/3 innings in a loss to the D-backs on Saturday.
Black said Richard continues to exhibit symptoms that are typically associated with an intestinal virus. A handful of other players and some members of the coaching staff have a brief run-in with a virus, but it's nothing like what Richard has experienced, Black said.
"It's bounced around, but no one has shown it to the degree that Clayton has," Black said. "So let's run some tests, see a specialist and get a handle on this."
Richard, who is 0-4 with an 8.54 ERA in six starts, has been replaced on the active roster by another left-handed pitcher, Tyson Ross, who was reinstated from the disabled list. Ross landed on the DL on April 20 with a left shoulder subluxation.
Ross won't replace Richard in the rotation. The Padres, who have two off-days over a five-day stretch starting Thursday, will likely have Edinson Volquez start Friday's game against the Rays.