Sir Bobby Charlton, Bryan Robson and Denis Law pay homage to manager's dedication and his rekindling of the youth policy
"Talk about Alex retiring has reared its head every three or four years and then been put back in its drawer and we've all waited another three or four years. But it's happened now, and I must admit there's been a torment in my mind ever since I knew positively about it a couple of days ago.
"I haven't spoken to him personally about the decision but I know he is very conscious of the fact that he is having a hip operation soon and has always felt that he has a duty to do more for his family. But Alex is not going away, he's coming on to the board, which automatically means he'll have a say about the new manager.
"Aside from all the success, Alex's biggest achievement has undoubtedly been his adherence to bringing youngsters through and making them part of an attacking team. Sir Matt Busby's philosophy was exactly the same and when Alex joined the club he immediately wanted to know about the contribution Matt Busby had made and what he had achieved at United.
"I honestly think he would get up in the middle of the night and go 300 miles in the car if he thought there was a player worth watching, someone he could sign for the club. And when it's a young player you've got to think of the parents and parents appreciate someone has got up in the middle of the night and gone 300 miles to see their boy.
"It is that sort of dedication, along with his ability, which makes Alex such a great manager and I know he still loves to work. But he's got to a stage where he feels he should appreciate his family more. That is natural."
"He's probably the greatest club manager there has ever been. Aside from what he has achieved at Manchester United, just look at his record with Aberdeen. Celtic and Rangers dominated everything in Scotland but he took a young team, won a European trophy and then also won the league. For a club like Aberdeen, that is an incredible achievement. He then came down to United, built five different teams, brought the youth system back and has gone on to do an incredible job. To win as many trophies as he did is unheard of.
"You could see from being involved at the club at the time how hard the boss and his staff worked to achieve everything they did. It doesn't just happen. It was tricky for the boss at the start but I didn't really fear for him. There was a lot of talk before the Nottingham Forest game [1990 FA Cup third-round tie] that he might be sacked if we got beat by Forest, but I spoke to Martin Edwards a few years later and he said the club never had any intention of sacking him. But the rumours were out there in the media at the time and I'm sure all United fans would say 'thank goodness' Mark Robins scored in that game and that the boss stayed.
"The way the boss has built the club over the years means there is a great structure in place for whoever takes over. Everything is there and the new manager will not have to make wholesale changes, just minor tweaks to the side. But whoever does come in has to be a strong character and stick to playing attacking football. That was the one thing about the boss; if we were drawing he was not happy and would make a change to try and win the game. He would never settle for a point and because of that the United fans have become used to winning."
"Someone said to me that Alex might be retiring and I honestly thought they were joking. I then turned around and looked at the TV and realised it was true. I was extremely shocked. I had been with Alex last week and he gave no indication that he was about to go; I even said a little whole ago that he wouldn't retire until he was 90. But he's gone. That's it.
"It would take me a long time to sum up what Alex has done for English football during his time at United. He has turned out to be one of the greatest managers the world has ever seen, if not the greatest. At the beginning of his time at the club there were problems and talk that he might even be sacked. But he won the FA Cup [in 1990] and then it was success all the way. People should also remember that before he came down to United, Alex had also enjoyed tremendous success with Aberdeen – he broke up the Celtic-Rangers monopoly – and it was then that you knew he was special.
"Alex has got a bit grumpy over the years but that has helped him become a great manager, and it should never be forgotten that he brought greatness back to United. He rebuilt the foundations that were lost after Matt Busby left and, like Matt Busby, Alex has concentrated on youth and giving young players a chance to shine in the side.
"He will be a very hard act to follow and there has to be the worry now that United will stop winning titles. I'm sure every United supporter is disappointed that Alex is leaving but I'm also sure that he knows what he is doing."