Head coach Mike McCarthy said Monday that Collins suffered a strained ligament in his left knee during the third quarter of the 33-22 win at Kansas City on Sunday.
The nature of the strain wasn't disclosed, but McCarthy said Collins would be out at least a couple weeks. Surgery won't be needed.
"It looked a lot worse on film," McCarthy said. "The evaluation (Monday) was actually positive. The MRI was positive, based on what (team doctor) Pat McKenzie said, because we all figured the worst the way he went down."
In steps Rouse, a third-round draft pick. He played most of the second half Sunday and received a positive assessment from the coaches, but it was only his second game in which he had playing time on defense.
Rouse missed an extended period early in the season because of a hamstring injury. McCarthy, though, feels the former Virginia Tech standout is up to speed in the scheme and can be effective in a full-time role.
"He's done a nice job. He's very bright," McCarthy said. "If you go back to the OTAs, he picked up the defense very quickly. It's his opportunity, and a lot of young players get their opportunity this way. I have no concerns about him. You don't really know until they play in the real games, and it's his time."
Rouse is an intriguing prospect as a safety because of his 6-4 size and athleticism.