As a starter, Dickey is 2-7 with a 6.03 ERA in 12 starts covering 68 2/3 innings. In nine relief appearances, he is 1-0 with a 1.80 ERA in 25 innings. Opposing batters are hitting .200 against him when he pitches in relief, compared to a .313 opponent's batting average as a starter.
Dickey had been scheduled to start Sunday afternoon's series finale against the Twins in Minneapolis. Riggleman said the Sunday starter would be announced later in the week.
"I know who it is, but we're not ready to announce it," he said, adding that right-hander Miguel Batista, who has made 19 starts this season, would remain in the bullpen.
One possibility is left-hander Ryan Feierabend, who has a 7-1 record and 2.16 ERA with Triple-A Tacoma. He started Wednesday night's game against Las Vegas in Tacoma and was lifted after 2 1/3 innings and 53 pitches, a clear sign that his next appearance could be Sunday at the Metrodome.
"We're going to look at some options, basically," Riggleman said.
Meanwhile, left-hander Erik Bedard continues to make progress on his comeback from a left shoulder injury. He extended himself Wednesday afternoon.
"He stretched it out today and he'll throw all three days in Minnesota, on flat ground and to a catcher [crouched] instead of standing up," Riggleman said. "Friday and Sunday will be a little more intense than the middle day.
"Hopefully, he comes out of weekend feeling good and we can schedule a legitimate bullpen session like he would have for a regular start."
Ideally, Bedard, who last pitched on July 4, would go on a Minor League rehab assignment, but there might not be enough time for that to happen. Tacoma plays its final game of the season on Sept. 1.
"It would be close and there might be a game he could go pitch in, but I don't know yet," Riggleman said.