Fletcher came through the ranks at United as a young player in the early to mid 2000s and said: "The scenes in the stadium weren't good.
"The chanting, the atmosphere really turned toxic for the first time."
Youngsters Brandon Williams, Daniel James and Aaron Wan-Bissaka all started for United, with Mason Greenwood coming on at half-time, and Fletcher says their development won't be helped by what is happening at the club.
"At the minute you can see with every setback these youngsters are just sinking," he continued.
"The scrutiny and the pressure that these kids are under - it was difficult for me coming into the team and I was surrounded by world class players left, right and centre.
"These lads don't have that support network around them and they are really finding it difficult.
"It might make a few of them, but at the same time it will probably break a few of them as well, which is really disappointing."