Austrian Manninger, who made 64 appearances during five years at Arsenal and played a key role in helping the Gunners win both the Premier League and FA Cup in 1998, died in a road accident near Salzburg on Thursday morning.
According to reports in Austria, Manninger's car was in a collision with a train on a level crossing on the Salzburg local railway in Nussdorf am Haunsberg.
Manninger, capped 33 times by Austria, was largely understudy to England international Seaman at Arsenal.
But he deputised for the former England No.1 in the 1997-98 campaign and delivered a string of impressive performances to help Arsenal – in manager Arsene Wenger's first full season – win the double.
"It is devastating news," Seaman, 62, said. "It is really sad and it is difficult to get your head around it.
"When I think of Alex Manninger, he was a fantastic goalkeeper for us. He came in and deputised for me during a really critical time to help us win the double.
"He did amazingly well. He was just a young lad when he broke into the first team. For a 20-year-old to come in and play for Arsenal in the way that he did was really special.
"I was out for quite a few games through injury. He played 13 times, kept a clean sheet in the key 1-0 league win against Manchester United at Old Trafford and saved penalties in our (quarter-final) FA Cup win against West Ham.
"The fans loved him. And I must admit it was touch and go as to whether I would get back in the side. He was a massive player for Arsenal."
Manninger, who began his career with RB Salzburg and joined the Gunners from Grazer AK in June 1997, left Arsenal for Espanyol in 2002.
He also had spells in Italy with Torino, Bologna, Siena and Juventus – with whom he collected a Serie A winner's medal – and joined Liverpool on a short-term deal in 2016 but did not make a competitive appearance for the Reds.
Seaman added: "He was a great friend, too. And like me was a fisherman and we spent may hours together on the lakes. My thoughts are with his wife and young family."
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