Halladay related the incident to last season when he got hit by a line drive in Tampa Bay, but said that he didn't have as much pain then.
The right-hander was moving around with the help of crutches on Saturday and was wearing a soft cast from his foot to just below his knee.
Halladay, who was likely to win the starting job at the 2005 All-Star Game, will no longer make the trip to Detroit and will instead stay in Toronto over the three-day layoff. His All-Star roster spot was filled by Boston's Matt Clement.
Halladay is expected to miss at least four weeks but is confident that he will be able to start playing catch in a week. He said he feels a throbbing pain occasionally when sitting still but that the only pain comes when he tries to put pressure on the leg.
"If I'm comfortable that it's not going to get worse, I have no problems pushing it as long as it doesn't aggravate it or set me back," Halladay said. "I'm going to trust [the trainers] on that and when we're at that point, I'm going to do all that I can."
Halladay was placed on the 15-day disabled list and outfielder Gabe Gross was optioned to Triple-A Syracuse. Right-handed pitchers Justin Miller and Brandon League were added to the roster and are available out of the bullpen for Saturday's game.
"It's huge. You look at a team and if you have one guy you really can't afford to lose or you hate to lose, it would be him," manager John Gibbons said. "Even though he's only going out there once every five days, you feel every time he's out there you've got a chance to win and we're pretty much doing that this year. It happens in sports. Guys go down and you've just got to regroup and somebody else has got to step up. We've just got to try to hold it together. There's no question that it's a big loss."
Halladay also missed time in 2004 with an injury. He was 8-8 in 21 starts a year ago.
The 2003 Cy Young award winner was 12-4 this season with an American League-leading 2.41 ERA in 19 outings for the Blue Jays, and was well on his way to another shot at the Cy Young.
"That's kind of how it goes sometimes," Halladay said. "The year that I did win it, [Mark] Mulder kind of had the same thing where he was having a better season than anybody else and it happened to him. It's part of the game. Health is a big part of that stuff."
Jake Veyhl is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.