"This afternoon, he came in, tried to take ground balls, tried to hit, wasn't going very well for him," Phillies general manager Ruben Amaro Jr. said. "And he wasn't going to be able to perform. We had said before if it got to the point where his knee got so debilitated and he couldn't perform, go ahead and shut him down."
Outfielder/first baseman Darin Ruf was recalled from Triple-A Lehigh Valley and entered Saturday night's game in the eighth inning.
Amaro noted Howard will have an MRI exam of his knee either Sunday or Monday, but for now, he is unsure if Howard will need surgery.
"We can't just think about the short term here, we have to think about the long term on Ryan as well," Amaro said. "I think we're going to be pretty conservative with him. If it needs surgery -- hopefully it doesn't -- if it needs surgery, we'll take care of it. If it doesn't, we'll try to rehab him and get him back."
Howard, who is in the second season of a five-year, $125 million contract, had been held out of the Phillies' starting lineup nine times in the last 44 games, and he did not start in four of the team's 10 games in its recent roadtrip.
Howard is hitting .266 with 11 home runs this season. He appeared in 80 of the Phillies' first 87 games, and manager Charlie Manuel called his absence "a big blow."
"His presence means something to us," Manuel said. "He's still a threat, he has  RBIs. We definitely need Ryan in our lineup. But at the same time, too, we have guys that can definitely perform better and help us win some games."
Ruf battled for a spot on the Phillies' Major League roster during Spring Training, but has spent the season in Triple-A. Ruf hit .266 with seven homers and 46 RBIs in 82 games for Lehigh Valley.
Amaro said Manuel will decide if Ruf will take over everyday duties at first base in Howard's absence.
"He's going to play some first base and he may even play some in the outfield," Manuel said. "We'll see what he can do."
Ruf didn't arrive at Citizens Bank Park until the fifth inning of the Phillies 13-4 loss Saturday, but he finished his debut 1-for-2 with a run scored.
He also played for the Phillies at the end of last season. The-26-year-old hit .333 (11-for-33) in 12 games.
"I want to help the team win any way I can, that's the goal here," Ruf said. "It's unfortunate it had to come up through these circumstances. But I still got to go out there and try to help them win as many games as possible."