The former Brentford and Leicester City boss was only installed as Keith Downing's successor at Whaddon Road last Monday, securing a 2-1 over Bristol Rovers in his first game but losing 2-0 at Millwall on Saturday.
"I've taken this decision after much thought and discussion with my staff," he said.
"Results have shown clearly that the first-team squad needs to be improved and, for me personally, we have far too many players here who aren't really threatening the first team.
"We need our first eleven improved, so I've spoken to all of the players personally. This is nothing to do with their attitudes because all of them have been absolutely first-class since I've been here.
"They're all good players and all good lads, but I do need to make some changes."