The DR Congo defender had scored for the Hammers in the third round against Barnsley and, this time around, he kept Gianfranco Zola's side in the competition after Stewart Downing's first-half effort had looked all set to deservedly send Middlesbrough into the last eight.
Both sides had been beaten by the Manchester clubs last weekend and, after beleaguered Boro's loss at City, they made a trio of changes as Julio Arca, Justin Hoyte and Gary O'Neil came in for Marlon King, Chris Riggott and substitute Matthew Bates.
That defeat at Eastlands had spelt an unlucky 13th Premier League match without a win for Gareth Southgate's side, who despite having seen off Barrow and Wolverhampton Wanderers in the previous rounds, have not won in the league since early November.
But with just a minute on the clock, his relegation-threatened, 19th-placed team had a great chance to forget about their lethargic league form when Jack Collison gifted possession to Downing but, once the ball broke to Adam Johnson, he blasted woefully wide.
Then with 11 minutes on the clock, Downing himself drilled a low 18-yarder across the bobbly Boleyn Ground turf and just an inch or so wide of the base of Robert Green's left-hand post, to the annoyance of the home fans desperately hoping that their team would click into gear.
Following last Sunday's defeat against table-topping United - the eighth-placed Hammers' only loss in nine matches - Zola had made just a solitary switch, as Luis Boa Morte replaced the rested Valon Behrami.
On the quarter-hour mark, Brad Jones was finally forced to muddy his gloves when he brilliantly beat away Mark Noble's point-blank shot and, shortly afterwards, James Collins' energetic 18-yarder was held at full stretch by the Australian.
But just as West Ham looked to be waking from their slumber, Boro deservedly took a 22nd minute lead when Collins failed to deal with O'Neil's right-wing cross and, with the ball eluding both Matthew Upson and Lucas Neill too, the stooping Downing ghosted in at the far post to nod the visitors ahead.
With Afonso Alves going it alone up front, the England wideman was enjoying the luxury of free, breaking role from the heart of Boro's five-man midfield and a first goal of the season was nothing more than the dangerous Downing deserved for his man-of-the-match performance.
Things got worse for the Hammers on the half-hour when Diego Tristan replaced top-scorer Carlton Cole, who hobbled away with an ankle injury and the agony just continued for Zola as Noble saw his bobbler clawed away by the groping Jones, only for Collison to blast the loose ball high into the chilly East End sky.
Just three minutes after the restart, Freddie Sears replaced the heavily-barracked Boa Morte and when the teenager hooked over from 20 yards, the claret and blue fans amongst the crowd of 33,658 looked resigned to a forlorn FA Cup exit.
As the hour-mark approached, Alves followed up a woefully wayward 15-yarder with a better, angled effort that scorched past the far post.
The Brazilian also forced Green into an excellent low save before the England keeper repeated the feat to deny Downing too.
In reply, the stumbling Tristan saw the ball plucked off his studs by the relieved Jones, while the heavily derided David Di Michele launched the ball into orbit from eight yards.
Those miserable misses were the catalyst for Zola's last turn of the cards as the Italian introduced Savio Nsereko in the hope that he could get the Hammers back on Wembley Way .
Sensationally, the Germany Under-21 international came up trumps with just eight minutes remaining.
O'Neil was booked for a deliberate handball on the left flank and that enabled the £9million signing to whip the consequent free-kick into the six-yard box where saviour Ilunga headed home to force that replay.