Cespedes DH'd nine straight games to rest his shoulder before returning to left field last Friday, only to last three innings before departing with pain.
"We have to be pretty confident," said manager Bob Melvin. "We have Thursday and Friday to figure that out, whether or not we feel like he needs pregame Friday to make a decision."
Cespedes was one of several hitters to step in against Dan Straily and Tommy Milone in an informal three-inning simulated game on Wednesday, but one of just a few to actually swing.
"His swing was a lot more aggressive," said Melvin. "I think yesterday, he was trying to ease into it, where today, you could tell right away his swings were much more aggressive. I think, hopefully, we're on a good path of getting him in the lineup somehow. We'll have a better indication tomorrow whether that's in left field or not.
"And with him, you can tell pretty easily. He'll try to grind through some things, but when you see the velocity he had behind his swing today, you can tell the shoulder wasn't bothering him."
Cespedes did not attempt to do any throwing on Wednesday. That will wait until Thursday, when he's expected to be limited to playing catch. Should he get through that exercise without any issues, the A's will look to lengthen him out.
In the event Cespedes is restricted to hitting duties come Friday, Melvin is expected to play Brandon Moss in left field -- "He can play anywhere," Melvin said -- and give Daric Barton the start at first base.
But ideally, the A's want a healthy Cespedes arm in left field.
When asked how honest Cespedes is with the staff when it comes to his health, Melvin smiled and paused for more than five seconds.
"Hmm," he began. "He wants to play -- let's put it that way. It's a combination of what the trainer thinks, how he feels, myself, all of the above."