Aston Villa have been stunned by the news that Martin O'Neill has quit as manager with immediate effect after four years in charge. O'Neill has resigned only five days before the start of the new Barclays Premier League campaign. Villa owner Randy Lerner insisted O'Neill had pledged his future to Villa in May after much speculation over whether or not the former Celtic boss would quit. But O'Neill has now decided to end his reign with reserve team manager Kevin MacDonald taking charge of Saturday's opener with West Ham. O'Neill said: "I have enjoyed my time at Aston Villa immensely. It's obviously a wrench to be leaving such a magnificent club. "I would like to pay tribute to the Villa players, my coaching staff and the Villa supporters for all the support and encouragement they have given both the club and me personally during my time as manager. "I wish them all the best for the future. I will obviously be assisting the club in the immediate short-term with regard to the handover of my duties." O'Neill has transformed the fortunes of Villa since replacing David O'Leary during the summer of 2006 with three consecutive top-six placings in the league. Lerner had backed O'Neill in the transfer market during that spell but it became clear this summer that the funds would not be available to help take Villa on to the next level. Lerner insisted that Villa would have to adopt a sell-to-buy policy and O'Neill was resigned to losing the services of midfielder James Milner to Manchester City. That followed on from skipper Gareth Barry also moving to Eastlands last year.