Both players are out of the Texans' training camp, with tackle Williams out with a knee injury, while Montgomery reported to the team out of physical condition and was placed on the non-football injury list.
When asked if it was frustrating, Kubiak told the club's official website: "Yeah, it is. Don't get me wrong now, there's nothing they can do about it.
"When you're injured, you can't go if you can't go. But it's just tough because it's been a long off-season and we're counting on those guys to push players on this team and be in position to help us.
"Right now, it's just not happening right now. Hopefully, we can get them going and hopefully we can get them out here as soon as possible."
The Texans reached the Divisional round of the NFL playoffs last season for the second year in a row.