Before the game Jack Ross had assured Sunderland fans that they need not worry about Will Grigg.
It had been a bruising week on the road for the January signing, starved at service at Adams Park and Oakwell.
He roared back in fine style, delivering his most important goal yet for the Black Cats.
They had battled back from a goal deficit but looked to be running out steam when the 27-year-old struck, bringing his side to within two points of Barnsley.
They still have a game in hand.
It had felt like a crucial afternoon for Sunderland even before the door was opened for them pre-match, given that they will not play again in the league for over a fortnight.
Barnsley had dropped points away to Doncaster Rovers on Friday night, but Sunderland started woefully and left themselves with a huge amount of work to do.
Just three minutes had been played when Walsall fired a long ball down the pitch. Tom Flanagan failed to deal with it giving away a foul that was whipped into the area, striker Josh Gordon firing home.
Sunderland looked sluggish, inviting their opponents into the game by giving the ball away in dangerous areas.
They did have the ball in the net from a free-kick, but the referee had already blown for a foul by Jack Baldwin.
Slowly but surely they began to dominate the game and Lewis Morgan had four efforts inside ten minutes that almost got his side level.
The first came as Aiden McGeady drove inside from the right wing, playing the ball out wide to Adam Matthews. He cut inside and found Morgan, but the winger fired wide.
He went much closer a minute later when McGeady found him in a central position, goalkeeper Liam Roberts having to palm his effort from the edge of the area onto the roof of the net.
The following corner then fell for Morgan, but his volley was well over.
McGeady had been moved onto the right flank for this game but his knack of fidning space in the centre looked like Sunderland's most likely route, and he again found Morgan in space, the Celtic loanee this time shooting low but wide of the far post.
The pressure eventually told for the hosts, but not before a significant let off.
Walsall broke and Zeli Ismail found himself in on goal. McLaughlin parried his effort away, but Luke O'Nien then had to do well to clear a cross away from trouble.
It did not take long for Sunderland to capitalise, a corner dropping for Lee Cattermole at the back post.
From a narrow angle he volleyed at goal, forcing it in seemingly via a defender.
It was almost two before half time when another Leadbitter set piece posed a threat, this time a free kick as Will Grigg drifted off his marker at the back post.
Having taken one touch to set himself, he forced a good low save from Roberts.
The home side were well in the ascendancy, and started the second half brightly, Leadbitter running onto a cutback and curling a shot just over the bar from outside the area.
Max Power headed over another precise corner from Leadbitter as the Black Cats pushed for a crucial second.
It was a positive start to the half but their momentum somewhat faded.
Ross was forced to ring the changes early, bringing on Lynden Gooch, Denver Hume and Charlie Wyke as the home side searched for more impetus.
Wyke hadn't yet taken a touch when within two minutes of his introduction, he helped his side take the lead with 20 minutes to go.
The ball was played into his feet and after gathering, teed up Grigg who took the centre backs out the game with a clever first touch into space. With his second he showed his class, firing back across the keeper on his weaker foot and into the bottom corner.
Walsall tried to force their way back into the game but in truth the hosts were rarely troubled.
Dan Scarr forced a save from McLaughlin at the start of five minutes stoppage time for the Scot but it was easy enough to deal with.
O'Nien then did well to head a good effort from Gordon clear as his side sealed a crucial win.
Thoughts can now turn to Wembley and the 40,000 strong invasion of the capital.
Sunderland know that when they return to league action, their fate is now firmly in their own hands.
Sunderland XI: McLaughlin; O'Nien, Flanagan, Baldwin, Matthews (Hume, 66); Cattermole (Gooch, 61) , Leadbitter, Power; Morgan (Wyke, 70), McGeady, Grigg
Subs: Ruiter, McGeouch, Sterling, Dunne
Walsall XI: Roberts; Devlin, Scarr, Guthrie, Leahy; Kinsella, Dobson, Edwards (Oteh, 87); Ismail, Taylor (Cook, 72), Gordon
Subs: Dunn, Fitzwater, Osbourne, Laird, Norman
Bookings: Leadbitter, 35 Guthrie, 37 Cattermole, 48 Gordon, 55 Gooch, 90
Attendance: 34,647 (647 away)