Arsenal manager Arsene Wenger believes that Manchester United manager Sir Alex Ferguson has achieved something "exceptional" during his time at Old Trafford.
Ferguson revealed on Wednesday that he will retire at the end of the season after 26 years in charge of the club, and Wenger has admitted that he can't imagine facing United without the 71-year-old in the opposite dugout.
The Frenchman told Arsenal Player: "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."
Ferguson's departure means that Wenger will become the longest-serving manager in the Premier League.
