Losing your starting QB is never ideal but if you look at it from a production standpoint, Winston's numbers so far this year seem manageable.
Not including last night, Winston has averaged this through 6 complete games:
15/25 186 yards 2 TDs .5 Int
Winston has also made some big plays with his feet extending drives. There's no way to replace Winston's arm talent but I don't think it's unreasonable to think Siemian could hover around those per game averages. But it's the same as it was with Winston, real simple, take care of the ball. If it's not there throw it away.