12:09 PM PT -- The Dodgers just revealed Freddie Freeman will somehow play in Thursday afternoon's game. He's slated to play first base -- while batting third in the lineup.

Freddie Freeman's evening at the ballpark Wednesday turned into a night at a local hospital ... as his wife says the MLB star raced from Dodger Stadium to a nearby emergency room to be with their sick son.

Chelsea Freeman said in a post on her Instagram page -- just before Freddie and the Dodgers took on the Giants in L.A. -- that Maximus, the couple's youngest boy, needed a prayer after he had come down with what doctors believed was transient synovitis.

The ailment -- which causes extreme pain and discomfort in the hip -- was thought to have stemmed from a viral infection the 3-year-old contracted during the Freemans' trip to the All-Star Game in Texas earlier this month.

Initially, Chelsea didn't seem ultra concerned -- though hours later, she informed her followers that things took a scary turn ... and she ended up needing to take the toddler to the ER.

She said Freddie joined them immediately after L.A. lost 8-3 ... and the couple stayed with their son in a hospital room until around 3:30 AM PT.

Thankfully, Chelsea said Maximus' prognosis now looks good.

"They ran a ton of tests & bloodwork panels that thank God they all came back negative," she wrote. "No infection or active virus."

It's unclear if Freddie will be able to play in the Dodgers' game Thursday after the late night ... first pitch is slated for just a few hours from now -- at 1:10 PM PT.