Red Sox Pitcher John Lackey Divorcing Wife Battling Cancer

Boston Red Sox pitcher John Lackey has filed to divorce his wife, Krista ... who is in the middle of battling breast cancer.

Lackey filed on August 30, according to court docs in Texas, claiming "the marriage has become insupportable because of discord or conflict of personalities." Krista and John got married in November, 2008.

Sources close to the family tell TMZ ... Krista underwent a double mastectomy back in March and underwent chemo as recently as June.

The divorce petition says John and Krista had a prenup. It also says Lackey has "separate property" he wants to keep for himself.

Lackey filed the divorce papers using only his and Krista's initials -- a tactic Tony Parker and Eva Longoria ... and Sandra Bullock and Jesse James also used in their Texas divorces.

It's unclear why Lackey -- who's struggled on the field all season long -- didn't wait til the off-season, about a month from now, to start divorce proceedings.

4:33 PM: The Red Sox just released a statement on Lackey's behalf -- telling us, "John Lackey is dealing with a deeply personal family issue, and it is one the Red Sox do not feel is appropriate for public debate. The Red Sox request that John and Krista’s privacy be respected."