Coates was injured in Saturday's practice, and an MRI confirmed the injury Sunday.
"When it happened, he didn't think it was that, we didn't think it was that," Richt said. "If he was going to get hurt, you wish he could have at least put his game gear on and done it in a ball game. He never got to play between the hedges, and that really hurt him."
Coates tore his left ACL as a senior in high school. He tore his right ACL twice in the lasttwo years, each time in preseason practice. The most recent injury was to his left knee.
Coates, who wants to be a football coach, will remain on scholarship and stay with the team as a student assistant coach.
"He's going to get a really great head start in (coaching)," Richt said. "He's still an inspiration to our football team. He's going to get all (the coaching experience) he wants, but it's a sad day for him and his family and for Georgia. He's just a beautiful kid who has not complained one bit."
A LITTLE BACKGROUND: Richt, who on Sunday issued an apology for complaining this weekend about the team's lack of an indoor practice facility, attempted Monday to explain why he was in a foul mood Friday.
The Richt version: He received two letters in the mail Friday that upset him, one of the team's position segments was late for practice, an individual player was late because he was moving his car from a tow-away zone ("So now I'm really starting to get hot," he said.), and another player had torn the shirt under his jersey intentionally ("That drives me up the wall," he said.).
By the time Richt gave his post-stretching pep talk, he was angry, he said.
"I just go off on everybody, and I'm basically giving them a fire-and-brimstone speech about tightening up the screws and getting ready to go," he said.
As soon as Richt told the team to hit the field, he was informed they couldn't practice due to lightning.
"I thought the guys' eyes were big because I was giving them a really good speech, but apparently the lightning was getting close," he said.
There are plans in the works, Richt said, for an indoor practice facility at some point in the future.
"I'm really not at liberty to say how far we've gone, but we've been working together for some time," he said. "We've formulated a very good plan for the future, and I'm excited about it, but nothing is a slam dunk."