Lewis Rankin's late strike rescued a point for Valley at Waterworks Field, after another below par performance from Carl Adams's men.
Town had trailed to a goal from the hosts just after the hour mark, which came with the double blow of losing left-back Lee Thomas to what turned out to be a broken leg in the process.
Thomas had been restored to the side after missing the previous two matches, with his right sided equivalent Keenah Rosser also back in after an illness had seen him sit out the last three.
Stefan Blake was preferred to Rankin in an advanced role in a third change to the starting line-up from the previous weekend's disappointing Vase defeat at Boldmere.
The game started slowly, with both sides unable to make any significant impact on the grassy but undulating surface.
There was a distinct lack of penalty box action, although Desborough keeper Chris Jones did keep out one strike from Danico Johnson early on, with Thomas also played in mid-way through the opening half - but he was subsequently unable to get a shot off.
Rugby keeper Matt Hill was relatively untroubled at the other end, with the hosts only creating a couple of half chances - which Lamar Parkes and Andy White failed to take advantage of.
Johnson and Romario Martin both had opportunities within a minute of each other as the first period neared its conclusion, but generally Valley's efforts lacked urgency and all too often they were forced sideways or backwards before their attacking attempts petered out.
Hill did make one decent save from Parkes ahead of the interval, before the deadlock was broken on 61 minutes when Desborough's Luke Stevens finished off a deep cross to the far post.
The home side looked the more likely to add to their total as Rugby continued to struggle to make any real headway on the game, with Hill doing well to keep out a decent effort from Jordan Henson.
The introduction of Rankin and Charlie Evans for the out-of-sorts Simeon Tulloch and Martin ten minutes after the opening goal did provide the visitors with a bit more impetus though, and Johnson went close on a couple of occasions.
Desborough sub Ty Clark put one high of the target, ahead of Town's equaliser with virtually the last kick of the game when Lewis Hayden's free-kick was played across the box by Evans onto Rankin - who side-footed the ball into the net.
Line-up: Hill; Rosser, Thomas (Edwards), Perkins, Francis; Hayden, Blythe, Blake; Tulloch (Evans), Martin (Rankin), Johnson. Subs not used: Kavanagh, Bunting. Booked: Perkins, Hayden, Blythe, Johnson.
Desborough line-up: Jones, Bradshaw, Stevens, Cooper, Adams, Boath, D.White, Arnold, Parkes, Henson, A.White. Subs: Law, Davies, Maisiri, Clarke, Turner.