Coming off surgery to repair ligaments in his left knee, Flacco played in two series spanning the first quarter. He took the Ravens (3-0) to a field goal in their opening possession and ended his performance by misfiring on a fourth-down pass from the Detroit 43.
Matthew Stafford played the first half for Detroit (1-2), completing 14 of 23 passes for 95 yards. He threw an interception and did not produce a touchdown.
The Ravens signed Watson as a free agent in March, hoping he would become one of Flacco's prime option this season. That plan ended when the 35-year-old tore his right Achilles tendon on the game's first play.
Watson appeared to trip while running a pattern and slumped to the turf. He was helped off the field.
In the second quarter, promising rookie running back Kenneth Dixon was stacked up on the goal line and left with a sprained knee. He came in with a team-high 66 yards rushing and added 41 more on six carries.
With Watson out, Flacco turned his attention toward wide receiver Mike Wallace, signed as a free agent during the offseason. Wallace was targeted six times, catching three for 37 yards.
After Jeremy Butler made an outstanding catch of a 25-yard touchdown pass from Mallett, Stafford threw his first interception of the preseason. After Dixon hurt his knee, Mallett scored on a 1-yard run for a 20-3 lead.
Copyright 2016 by The Associated Press