Syers struck in stoppage-time to defeat Oldham on Boxing Day and he was at it again after just 58 seconds at Deepdale to put the visitors ahead.
David Cotterill doubled Doncaster's advantage with a long-range free-kick after 30 minutes before Billy Paynter sealed the win from the penalty spot in the second half to leave Preston without a win in eight npower League One games.
Rovers took the lead when Cotterill's low corner was hooked in via the crossbar by Syers at the near post.
It was 2-0 on the half-hour mark when Cotterill's free-kick from wide on the left evaded everyone and nestled in Thorsten Stuckmann's net.
Will Hayhurst and substitute Anthony Elding both shot wide before Hayhurst was penalised for bringing down Kyle Bennett in the area and Paynter sent Stuckmann the wrong way from the penalty spot after 76 minutes to seal the win.