Must happen/Can't happen against Seattle

SANTA CLARA, Calif. -- Here is a look at what the San Francisco 49ers must do and can't do Sunday at home against the Seattle Seahawks:

Must happen: The 49ers must have success against the Seahawks’ secondary. One of the best units in the NFL, the Seahawks are playing without suspended cornerbacks Walter Thurmond and Brandon Browner. The Seahawks’ secondary bottled up the 49ers in the past two meetings between the teams. But with Michael Crabtree back and the issues surrounding Seattle, the opportunity is there the 49ers. They must take advantage.

Can’t happen: The 49ers can’t allow Seattle to jump out to a big lead. The Seahawks have blown out the 49ers twice in the past calendar year. If the Seahawks jump out to a 14-0 lead, it will get in the heads of both teams. The 49ers can’t allow Seattle to get an early edge.