According to his Wiki..
"While preparing for his semifinal match vs. Mac Danzig, Arroyo sustained a rib injury and had to withdraw from the competition, prompting Dana White to select John Kolosci as his replacement. Matt Arroyo went on to defeat John Kolosci at The Ultimate Finale 6 via submission (armbar) at 4:42 of the first round.
In Matt Hughes TUF 6 Episode 12 journal, Dana White confirmed to Matt Serra and Matt Hughes that the doctor found no such injury to Arroyo's rib."
But I don't remember any of this, it was like 3 years ago and I wasn't paying too much attention to TUF at that time.