The Patriots are down to rookie Jacoby Brissett as the only other quarterback on the active 53-man roster with Garoppolo suffering a sprained right AC joint in Sunday's 31-24 win against the Miami Dolphins.
"I'm not sure exactly what the situation is going to be, I think a lot of that will be determined later today relative to Jimmy's whole situation and what it's going to be. I'm sure if he can't play or back up, then we'll be going through that process today," McDaniels said on sports radio WEEI's "Kirk and Callahan Show."
"I haven't heard anything yet this morning exactly what that's going to be, but I'm sure we'll be on that quick. It's a short week, we're going to have work hard to get somebody ready to go in case that does happen, if that situation does come up, we'll have somebody ready to roll."
Garoppolo, starting his second straight game for the suspended Tom Brady, had already thrown three touchdown passes when he was injured. Garoppolo will be assessed day-to-day but likely will be unvailable when the 2-0 Patriots host the 2-0 Houston Texans on Thursday night.
Brissett, who was taken in the third round out of North Carolina State, made his NFL debut and led the Patriots on a touchdown drive on the first drive of the second half, with LeGarrette Blount scoring on 9-yard touchdown run.
McDaniels praised Brissett, who was 6-of-9 passing for 92 yards, on his approach.
"Jacoby is really a calm individual," McDaniels said on WEEI. "He's been that way since we brought him in for his interview and we tried to put him under pressure when we first brought him in here before the draft, and he didn't flinch then. He's a guy that is very difficult to fluster.
"He's young, he doesn't have much experience obviously, and he's learning every week and every day. But I thought his poise and composure yesterday showed. I didn't feel like he was rattled on the sideline when it was his opportunity to get in there. I felt like he handled the huddle, the line of scrimmage and some critical situations well."