No manager in the league's history has done what he has with the budget he has used. Consistent top 4 finishes, FA Cup success, League success. He has brought Arsenal into their most successful stage in their history. He has got them into possibly the finest stadium in the country in those years too. He's built the team from scratch and unearthed some of the finest players to grace the Premier League, not to mention the entertaining football they played whilst doing this. In 8 years Chelsea spent 351m+ City 400m+ Liverpool 150m+ and United 130m+ in comparison he spent around £40m. He won a trophy much sooner than Ferguson did, he revolutionised both the onfield and offfield ways of English football and essentially put the standard in place that we see today. This is everything from the football played to the off field behaviour of players (re: fitness, alcohol, food etc.).
Well, that's a quick explanation, I'm sure you could find many articles about the topic on the internet.
To say he is not in their league is quite frankly ludicrous.