The Saddlers have now collected just two points from the last five games and appear to have lost any chance of winning a play-off spot.
Crewe, meanwhile, eased fears of relegation with another valuable away point following a 61st minute strike from Nicky Maynard for his eighth goal of the season.
Walsall's alarming decline in recent weeks was emphasised with another poor first-half display when only veteran Tommy Mooney displayed any talent to lift the Saddlers out of the doldrums.
The only highlights of a goalless first period came from Mooney's attempts to open his account. Three times he produced goalbound attempts, only to be thwarted by three outstanding saves by Ben Williams.
Crewe's lowly league position was emphasised by their lack of penetration despite Maynard's good strike.
There was little real danger posed by their attack and their only real threat came late in the half when Dean Morgan conspired to guide his shot wide of the far post.
With so much at stake for both teams it was alarming that there was no determination from either side to break the stalemate in the first 45 minutes.
Julian Baudet was the strong man of Crewe's defence, while Anthony Gerrard took top marks for the Saddlers.
The personal battle between Mooney and Williams continued in the second half when the Crewe keeper flicked a shot over the bar.
It sparked off a spell of attacking pressure from Walsall but it ended in a counter move which finished with Maynard lashing a cross-shot past Clayton Ince following a pass from Tom Pope.
Substitute Alex Nicholls raised Walsall's hopes of an equaliser, but his low raking shot went just wide.
However, with the Saddlers heading for their third successive home defeat Roper came up with the goods. He moved up for Lee Holmes' corner and forced the ball home despite Williams' bid to make another save.