Former Boston Red Sox third baseman Will Middlebrooks posted a heartfelt message to social media on Tuesday in honor of his "best friend," former NFL quarterback Ryan Mallett, who tragically died in an apparent drowning in Florida.

"I lost my best friend today," Middlebrooks wrote in a tweet which included a series of pictures of the two over the years, including images of Mallett from his time with the New England Patriots.

"Someone who has stuck by my side since we were just kids. The most competitive, hard headed, fun loving person I ever met. Nothing prepares you for this. I was lucky to have him for as long as I did. Rest in Love One-Five. We love you."

TOM BRADY REACTS TO RYAN MALLETT'S DEATH: 'WE LOST A GREAT MAN'

According to the Okaloosa County Sheriff’s Office, first responders were called to a beach in Destin, Florida, at around 2:12 p.m. after a man, later identified as Mallett, was pulled out from the water and was "not breathing." 

"A group of people in the water near the second sandbar had reportedly been struggling to make their way back to shore," law enforcement said in a news release. 

Despite the efforts of the first responders, Mallett was later pronounced dead at a nearby hospital. 

CLICK HERE FOR MORE SPORTS COVERAGE ON FOXNEWS.COM

Middlebrooks also shared a lengthy message from his mother, Julie Procell Middlebrooks, on Wednesday where she remembered Mallett as a "son." 

"I have cried so many tears, yet I know my pain is nothing like your family's," she wrote in the post. "I'm still crying ugly tears right now. You called me mom, and I called you son." 

"Every single birthday I heard from you. Every single one. ‘I love you, mom.’ It's going to hurt so bad not to hear that from here out. But I know you loved us all. I love you, Ryan, and I have for a very long time. Through the good times and the bad, we always considered you partly ours, just like your family has always considered Will part of theirs (Will much preferred your mama's breakfast over mine)."

Tributes for Mallett poured in on social media Tuesday after news of his death broke. 

"We lost a great man," Tom Brady said of his former teammate. "Thank you for everything Ryan. … Praying for the Mallett family and all their loved ones tonight."

Mallett was a standout quarterback at Arkansas before the Patriots selected him in the third round of the 2011 NFL Draft. He was with the Patriots until the 2014 season, when he joined the Houston Texans. He was later traded to the Baltimore Ravens before he came a free agent in March 2018.

Fox News’ Ryan Gaydos contributed to this report.