49ers score late to stave off Packers’ upset efforts, 24-21

History said the 49ers were in trouble when they entered the fourth quarter trailing.

The 49ers were 0-30 under Kyle Shanahan when trailing by seven or more points after three quarters, losing by an average of 14.3 points. Quarterback Brock Purdy had led only one fourth quarter comeback in his career, a 37-34 overtime victory over the Raiders in Week…