Arsenal manager Arsene Wenger says English football will not be the same without his old rival Sir Alex Ferguson.
The veteran Manchester United manager announced this week he will retire after more than 26 years at the helm at Old Trafford.
Wenger and Ferguson have played out many duels over the years, most notably when their two clubs were challenging head to head for the Premier League title in the late 1990s and early 2000s.
Wenger, who will become the longest-serving manager in the country after taking charge at the Gunners in 1996, told the club's website, "I would just like to pay tribute to an unbelievable achievement and a fantastic career. Basically the achievement is immaculate, when you look at the whole structure and consistency of the achievement. It is, of course, something exceptional.
"It is difficult to imagine English football without him, but it's now a reality and a fact. Of course the next manager has to fill in and show he has the dimension to do that. It is a big task for the guy who comes in."
The man to take over at United is Everton's David Moyes, another long-serving boss after 11 years at Goodison Park.
Wenger expects Moyes' task in filling Ferguson's shoes to be a difficult one.
"What you can say still is that Manchester United are commercially and financially one of the strongest two or three clubs in the world, and that is still a good basis to start when you come in because you know the players are there, the team is there, and the potential is there," Wenger said. "They have developed very well and they are in a very, very strong position to deal with the problem they face.
"But of course it's a great void to fill for Manchester United, because the charisma and personality disappears suddenly in a club which has been dominated by it for such a long time. It's not an easy task to replace a person like that."
Manchester City manager Roberto Mancini said it was an honor to compete against Ferguson, and called his crosstown rival the best manager of his generation.
"For me, it has been a great honour and pleasure to compete against Sir Alex for three years," Mancini said. "It was a great honor to beat him in Old Trafford and I wish him good luck for his future. ...
"I don't think there will be another manager like him. It is difficult to say Sir Alex is the best (ever) but he is the best in the last 27 years."
Former England manager Fabio Capello was also gushing in his praise of Ferguson, but admitted he had been taken aback by the announcement due to a conversation he had with the Scot earlier this season.
The Italian said: "I met him a few months ago at the England Sports Centre and he told me he would carry on, so the news of his retirement surprised me.
"I called him yesterday and he told me he has decided to go now because there is too much stress nowadays. Always having to win something is very challenging, and he told me he wanted to go around the world with his wife. His secret is amazing. To have that strength and ability to change things and improve every year, and always have the same desire to win."
West Ham boss Sam Allardyce, who was still playing when Ferguson began his Old Trafford reign, cited the 71-year-old as a major help when he was cutting his managerial teeth in the 1990s.
"He has had huge influence on me," he said. "I got to know Sir Alex when I was at Notts County when I was invited on to the LMA (League Managers' Association) committee. "To get to the Premier League with Bolton and get to compete against him and build up a pretty good friendship with him... He has been a huge influence on me.
"I have been managing for 20 years now and a lot of that has been phone calls when you are in a pretty dark place in this game as a manager. You need some advice and you need to know who to turn to, and that solid advice would always come from Sir Alex if you rang him."
Harry Redknapp has been doing battle with Ferguson in the Premier League for the best part of 20 years and believes his retirement will leave a big hole in the game.
"He will be a great loss to the league and football," he said. "He is a fantastic manager, but a top bloke as well who always had time for young managers if they phoned up -- no matter what level they were at. He was always there for anyone and he was a big driving force behind the LMA. That shows you what he was about.
Information from Press Association was used in this report.