Arsenal manager Arsene Wenger claims English football will not be the same without his old rival Sir Alex Ferguson.
The veteran Manchester United manager announced this week he is to 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, http://www.arsenal.com: "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.
He said: "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.
"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."