It's the 4th and 5th metatarsals the bones in his left hand that connect the ring finger and pinky to the wrist it's something that he shouldn't be allowed to play through so it heals properly. I'm sure it would feel worse if it was his bottom hand on the bat.
Ben Cherington on Oneil Cruz:
Injured the hand on Saturday vs. Atlanta. X-Ray in ATL was initially negative...standard protocol is x-ray first, if negative, then you just follow the player to see if symptoms are improving. Cruz said he was feeling better, and told the team he could go on Tuesday. Once he started hitting on Tuesday, 'he felt it more acutely,' and then a CT scan/MRI was taken...the more specific test then revealed the fracture.
Cherington says 4-6 weeks is the timeline for Cruz, BUT... "knowing Oneil, he can play through pain. If he can get back out their quicker, he will."