The Portuguese grew increasingly critical of his players in the media last season, which culminated in him being fired from the role in December.
Speaking in Singapore ahead of Saturday's clash with Inter Milan, former United captain Robson said: "If you're not doing as well as you want you get uptight and say things in the media and wish you hadn't said that.
"Jose Mourinho did a great job when he came in. Then things weren't happening around the club which he wanted. He started criticising the club and the players and you can't do that.
"When you are a big club like Manchester United you can't criticise the people you're trying to get the best out of. That's where he fell down at Manchester United – criticising players in the media."