I think he can be adequate. I don't expect him to play very well, don't expect Garoppolo type of results (hell, before proving himself, I didn't know HE could get those results). Garoppolo has 3 things over Brissett:
1) Incredibly quick release, Brissett's release is not nearly as quick, pretty elongated release. 2) Garoppolo is already good at reading NFL defenses and looking off his 1st and 2nd targets, Brissett looked very good in preseason games, but I did notice him lock in on his target. 3) Garoppolo has 3 years in this offense, and he showed how well he knows it, Brissett has a couple of months.
I would be shocked if we win this game, less shocked if Garoppolo was healthy, but extremely shocked if they can win this game with Brissett. A few things have to happen, 1) NO turnovers, have to be mistake free, can't give short fields and free points. 2) Defense HAS to show up, they've been horrible in the 2nd half, have no confidence this defense can protect a 14-21 point lead. They can't afford to be that pourus if they want to win this game.