The length of the absence is still to be determined, a club spokesman said a few hours before the Phillies were to close a three-game series, but had nothing to do with any physical problems.
Later on Wednesday, the club issued an official statement.
"There has been a tremendous amount of tornado activity near Roy's home in Mississippi, resulting in several tragic deaths and significant devastation to the area," the statement reads. "Because of this, Roy is concerned about his family's well-being. He has chosen to take time to make sure there wasn't significant damage to his home, but more importantly, to make certain that his wife and children are OK.
"It is almost a year to the day that his parents' home was destroyed by a tornado, which has heightened his concern about the current situation.
"We are hopeful that he will rejoin the team for his next start, but will take that day by day."
Oswalt had left Tuesday night when the clubhouse doors opened and was not there to respond to queries about a possible recurrence of back spasms that plagued him a few starts ago.
"He was all right," Phillies manager Charlie Manuel said when asked after that game if Oswalt was having any physical problems. "His stuff wasn't good. His stuff wasn't crisp. His velocity was down. I figured there was no sense leaving him out there."
Any question about that back?
"Ah, no," Manuel said. "He didn't say anything about that."
Oswalt allowed five runs on six hits with a walk and wild pitch after retiring Arizona in order in the first inning.
Oswalt came into the game with a sparking 1.88 ERA and had only allowed five earned runs in his first four starts, which produced three victories and a no-decision in the April 15 game against the Marlins when he came out because of the back problems. After his first loss of the season, his record now stands at 3-1 and his ERA at 3.33.
Brian Schneider, who caught on Tuesday to give Carlos Ruiz -- in the midst of a 0-for-20 slump at the plate -- a rest, said that Oswalt's usually reliable dancing changeup was hanging over the plate.
"His stuff was there," Schneider said. "In the last couple of outings, his changeups were down. There were a lot of swings and misses -- guys chasing it with two strikes. They were still swinging at it, but they were up in the strike zone and they put the ball in play. They found a lot of holes tonight."
Schneider reiterated that he didn't think Oswalt was throwing with any pain.
"He gave me no indications of it," Schneider added. "That's something he's going to have to answer. Prior to the game, talking to him, his bullpen [session] was great, so that's nothing I can answer for you."
Barry M. Bloom is national reporter for MLB.com and writes an MLBlog, Boomskie on Baseball. Follow @boomskie on Twitter. This story was not subject to the approval of Major League Baseball or its clubs.