Mike Trout is opening up on the passing of his brother-in-law ... saying, "You brought so much happiness and joy to my life every time I was with you."
The best player in baseball stepped away from the Angels a week and a half ago to mourn the loss of his wife's 24-year-old brother, Aaron Cox -- who was a former baseball player himself.
No details on the death have been given ... but Trout's message Thursday emphasized how much Aaron meant to the two-time MVP.
"Aaron, words can’t describe the emotions that I’ve been feeling the past week and a half.