Alexander takes up job with Dons' League Two rivals Bradford City
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He takes with him assistant Chris Lucketti, who followed Alexander out the exit door at Stadium MK too.
“It is really good to be here," he said. "I am very excited about the role, the potential of the team and the club.
“Me and Chris have been here quite a few times as a management duo competing against Bradford City, but it will be good to be on the right side, now.
“When the stadium is full and the crowd is behind the team, it is a really difficult venue to come and play at, so we have to make it a tough place for everyone else to visit.
“Joining a club, you have to look at the short-term improvements you can make, but also look at the potential of what you can do. Here, for me, that scope is endless.
“I am really excited for the weekend. You will not see me sat down in the dugout too much, but we know what this venue is like for a game of football, and we are looking forward to getting our supporters behind this team and bringing them excitement.”