Netzer said that the Blaugrana are reliant on the Argentina player and if Bayern Munich can stop him over two legs then they will progress to the final on May 25 at Wembley.
"I've always said Messi is, for me, 50 per cent of this team," he told TZ. "He is the best player in the world, but additionally there are good players, Xavi, Iniesta, around him, all world-class players.
"But this year Messi appears to be more than 50%. If they succeed in stopping him Bayern will survive this round.
"Yes, you can 'turn him off'. The game Milan played at home showed that. Messi was non-existent for 90 minutes, because they fought to mark him as a team. More inexplicable was the return game, where Messi was the all-important man and Milan had nothing under control.
"He must always be surrounded by two or three players and must be blocked at an early stage. It sounds simple but it is not in reality."
Netzer added that the tie is an even contest with both sides on top of their game and neither can be considered as likely winners or clear underdogs.
"There are no favourites in this game," he added. "The biggest compliment I can pay Bayern is the fact that Barcelona are not necessarily the favourites. Barca is probably still the best team in the world, however."
The first leg of the semi-final tie takes place on Tuesday night at the Allianz Arena, with kick-off at 19.45BST.