I will admit right off the bat that these are entirely cherry-picked stats. After the game, FOX's Curt Menefee apparently pointed to points allowed, sacks and first downs gained to describe this as the 49ers worst offensive performance since 1964. I did a little bit of research over at Pro Football Reference, and confirmed that. Well actually, it is 1963, not 1964.
The San Francisco 49ers offense had a field goal and eight first downs, and gave up six sacks. There are plenty of stats to consider in a loss, but points, first downs and sacks are often going to cover a lot of the issues a losing team might have.
I decided to expand the search a bit, looking up when the 49ers had three or fewer points, ten or fewer first downs, and gave up at least five sacks. According to PFR, the 49ers did that in a 23-3 loss to the Seattle Seahawks in 2007, and a 26-3 loss to the Detroit Lions in 1963. Those were not good 49ers teams.
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In 2007, Alex Smith got the start in that game, and left with his infamous shoulder injury after one pass attempt and one rush. Trent Dilfer came on in his place and completed 12 of 33 passes for 128 yards and two interceptions. Frank Gore rushed 16 times for 79 yards.
In 1963, Bob Waters got the start in Week 2. He completed 8 of 19 passes for 52 yards, while throwing one interception. Lamar McHan, who started most of the games that season, took over eventually and was 2 of 3 for 11 yards. Don Lisbon led the team with 53 yards on 12 carries.
So yea, consider this a little more context as to how awful the 49ers offense was on Sunday.