MLB probes social media posts on Rays' Franco; he sat out Sunday's game.
Team acknowledges posts, cooperates with MLB investigation.
Content of posts not disclosed by Tampa Bay.
Basabe debuts at shortstop in Sunday's game, aware of call-up Saturday.
Franco present at start, departs during fifth inning.
Tropicana Field offers Wander Franco Snapback Hat promo for young fans.
Franco's absence felt in Rays' 9-2 loss to Cleveland.
Details on posts and investigation status awaited.