It was rough luck on the home side, who had done most of the attacking in a match lacking in quality and goalmouth action, but the visitors grew stronger as the game progressed and Ajose delivered the suckerpunch three minutes from time to end Doncaster's unbeaten start to the campaign.
The Red Devils were seldom seen as an attacking force in the first half and relied on the long ball for their attempts at goal with Nicky Adams going close following a breakway move in the 43rd minute.
Rovers had most of the play, but failed to capitalise on their chances with Billy Paynter firing wide and heading over in quick succession.
Doncaster were unable to step up the pace after the interval although Dave Cotterill had a 20-yard snap shot well saved in the 53rd minute.
It was Crawley who finished the game on top with Ajose forcing Gary Woods to save for the first time in the 86th minute before snatching the all-important goal seconds later when he deflected an effort from Josh Simpson into the net.