"Tad has been in a little bit of a funk the last 10 games or so," Black said. "I got the sense he needed to take a little step back ... not put too much pressure on himself to get through this little slump. One way to do that is to move a guy down in the order."
Iguchi, who entered Monday's game hitting .213, had hit second in the Padres' lineup in each of his 18 starts this season.
Paul McAnulty got the start in left field and hit second. Another benefit to the change in the lineup, Black said, was getting another left-handed bat in the lineup against Astros starting pitcher Roy Oswalt, who entered the game 1-3 with a 6.65 ERA.
"When you look at our options, he gives us the best option," Black said of McAnulty, who is hitting .250 but had his first home run of the season Saturday. "He's been swinging the bat OK."
Left-handed batters are hitting .410 against Oswalt this season.
"When you couple that with some left-handed hitters at the top of the lineup against a top right-handed pitcher, we think it has a dual benefit to both players and the club," Black said.
Iguchi was hitting .313 after collecting two hits against the Dodgers on April 11. But he is just 1-for-27 since that game with six strikeouts, including going hitless in seven at-bats in the 22-inning loss to the Rockies on Thursday.
Iguchi, a career .273 hitter, has primarily hit second in his Major League career. Black didn't sound as if moving Iguchi to eighth in the lineup would be a temporary move.
"We like the way that looks," he said.
Corey Brock is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.