Mike Trout Posts Heartbreaking Message After Brother-In-Law's Death

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. You were more than just my brother-in-law... you were my best friend," Trout wrote.

"You made such an impact on my life since the day I first met you. You were an amazing person inside & out that showed us all how to live life to the fullest. Seeing and hearing about your impact on other people are all things that made me a better person every single day."

Trout says the two shared plenty of memories together ... including hunting, golfing and fishing trips.

"You were a great kid and it kills me to know that you are gone... I can’t thank you enough for what you have meant to me over these last 11 years. I know you will be watching over me now that you are gone, I promise to take care of your sister and watch over your whole family."

"Heaven got a great one and I know I will see you again someday... I love you bro!!!!"