Gelof (oblique) was reinstated from the 10-day injured list and is starting at second base Tuesday against the Astros.
The 24-year-old has been sidelined nearly three weeks by an oblique strain and is back with Oakland after a two-game rehab assignment with Triple-A Las Vegas. Gelof had a slow start to the campaign with a .597 OPS through 24 games but should operate as the Athletics' primary second baseman now that he's healthy.