SANTA CLARA, Calif. -- Here is a look at what the San Francisco 49ers must do and can't do Sunday at Arizona:Must happen: Get the ball to Vernon Davis. When the 49ers beat Arizona 32-20 in October, Davis took over. He had one of the great all-time days by a tight end in NFL history. He had eight catches for 180 yards. The Cardinals have struggled covering the tight end this season. The 49ers have to exploit it, especially after Davis was held catch-less against Atlanta.