The Portuguese striker replaced Argentinean Ruben Dario Larossa in the 71st minute and took only 60 seconds to notch his fourth goal of the season.
It came from a quickly taken free-kick by Kris Taylor to Matt Fryatt who in turn fed Mark Wright and his cross was met at the far post by Leitao's firm header.
Yet Doncaster had dominated for much of the match and as early as the fourth minute James Coppinger had a great header from a free-kick by Michael McIndoe which flew just over.
At the other end Wright hit the angle of bar and post with a header from a cross by promising teenager Ishmel Demontagnac.
Rovers continued to be the better side and striker Jonathan Forte should have put them in front moments before the interval but his shot was saved by keeper Joe Murphy.
Murphy, returning to Walsall for a second spell having been on loan at the Bescot in the latter part of last season, kept Rovers at bay with a series of saves early in the second half and Paul Merson's men were also indebted to central defender Ian Roper who made some superb interceptions on his return to the side.
In the 79th minute Doncaster manager Dave Penney made a double substitution, bringing on Lewis Guy and Leo Fortune-West for skipper Neil Roberts and Coppinger.
Both substitutes might have got on to the scoresheet in the 81st minute but failed to take advantage of a fine cross by Forte.
In the 89th minute Leitao should have added another goal only to miss from a pass by Michael Standing but Rovers will rue the fact that they squandered so many good chances.