"It's still a day-to-day process," Foster said. "I'm going to go tomorrow, see how it feels and just keep working.
"Yeah, every day it feels that much better, so I'm just going to keep going."
"I think it could if he shows where he's at tomorrow and we feel very good about that and end up working him pretty hard Saturday morning and making sure he took another step forward," Kubiak said. "That's something that can always take place in that last 48 hours, but we'll see where he's at tomorrow."
Foster, the 2010 NFL rushing champion, injured his left hamstring early in training camp, then aggravated it during an Aug. 27 preseason game against the San Francisco 49ers. He has been limited in practice since, and he said Wednesday that he might not know until just before kickoff if he'll be ready to go.
Before Thursday's practice, though, Foster indicated his rehabilitation was going smoothly and that he expects to play.
"The hamstring is coming along," Foster told Yahoo! Sports Radio. "It feels really good. All signs point to me playing on Sunday."
"Like I said yesterday, we need to be at our best as a football team to win this game, and he puts us at our best," Kubiak said of Foster. "But at the same time, if he can't go, if the trainers say that 'No, he can't go or it's not smart,' then we have to listen to them. Everything we're doing or have been doing has been working toward Sunday, and it's Thursday. You keep your options open, but obviously, he wasn't able to practice today."
Foster had a career day against the Colts in last season's opener -- with a franchise single-game-record 231 rushing yards and three touchdowns -- but he isn't in a rush to return for one game to start off a long season.
"With hamstrings, it's not something you can push through," Foster said. "When you try to push through, it's just going to re-occur. It's not like a knee. Like with my knee, I could just push through it because you're not going to mess it up too much more. With a shoulder, you can just push through it. You can't just push through it (with a hamstring). It's something that has to heal completely before you can come back completely."
The Associated Press contributed to this report.