Cooper (back) was activated from the 10-day injured list and is starting at first base Friday against the Nationals, Christina De Nicola of MLB.com reports.
The 30-year-old has been sidelined nearly three weeks due to a lumbar strain, but he'll rejoin the Marlins after appearing in a pair of rehab games with Triple-A Jacksonville. Cooper should see semi-regular starts between first base and the corner outfield spots, and he could have an expanded role in left field while Corey Dickerson (foot) remains sidelined.