In the letter, typed in Comic Sans font, Gilbert called James a "coward" and mocked the many nicknames and catchphrases attached to James, such as "King James," "The Chosen One" and "Witness."
The letter remained on the Cavs' website for the past four years until the team removed it earlier this week.
Although James recognizes that informing Cavs fans and Gilbert, who was told of James' decision to leave Cleveland in a telephone call five minutes before it was announced, on national television was a mistake, he is still wrestling with whether to return to play for Gilbert's team.
"If it wasn't for that letter, this would've been done a while ago," one source said.
A Cavs source downplayed the letter to ESPN's Josina Anderson on Friday morning, saying that there was "no indication from them to us at this point either way" on the letter playing a part in holding up LeBron's decision.
Although there have been rumblings about Gilbert and James clearing up their differences in the past, no such discussion or meeting has been confirmed. What is known is that Gilbert has never issued a public apology to James.
Sources close to James said Thursday that the four-time MVP wanted to speak with his family before making his decision. Many around the NBA have wondered what there was left to talk about, but James' concerns about the Gilbert letter is a plausible explanation.
While league executives wait for James to make his announcement, many refuse to believe that he'll return to Cleveland to play for Gilbert. Some even believe it would be hypocritical after how powerfully James spoke out against Los Angeles Clippers owner Donald Sterling after racist comments by Sterling were made public in May.
James was the first player to speak out against Sterling, saying he had no place in the NBA.
Gilbert Letter An Issue For LeBron
Chris Broussard and Marc Stein discuss the report that Cavaliers owner Dan Gilbert's 2010 letter regarding LeBron James may be one of the reasons James is taking so long to make a decision about whether to play in Cleveland next season.
Gilbert Apology Important?
'SportsNation' debates whether a public apology by Dan Gilbert to LeBron James is important.