An afternoon which surely extinguished Liverpool's hopes of Champions League qualification raised far more questions than answers. Where, for instance, might Kenny Dalglish's team be without Craig Bellamy? Why do Andy Carroll and Stewart Downing not feature in more Anfield starting XIs? Is Luis Suárez losing interest?
Should Sunderland's Jack Colback be regarded as mildly or seriously underrated? Is Nicklas Bendtner's knee injury really that serious? How bad a signing has Charlie Adam been for Liverpool?
Ponderous in possession, horribly prone to losing the ball and constantly fouling, Adam disappointed in a midfield department in which Colback, the academy graduate deputising for the suspended Lee Cattermole, quietly shone alongside the much-improved Craig Gardner. There was a time when Colback was kept out of Sunderland's team by Jordan Henderson but here the right-sided Liverpool midfielder's industry failed to disguise an ordinary performance.
"I've told Jack I've got more belief in him than he does at the moment," Martin O'Neill said. "Jack's a very fine player, all he lacks is self-belief. But he's shown he can mix it with the Premier League's best including Steven Gerrard. Perhaps Jack should listen to Nicklas Bendtner ... Or perhaps not."
Bendtner, borrowed from Arsenal, has spent much of this season infuriating those Wearsiders who fret about the inordinate amount of time he spends drifting around outside the box.
Here, though, he excelled, providing the inexperienced Liverpool centre-half Sebastián Coates with a tough workout before hooking Sunderland's second-half winner home from close range after Fraizer Campbell's 20-yard shot had rebounded off both the back of Pepe Reina's head and a post.
When Bendtner was carried off after an abrasive late challenge which prompted a booking for Martin Skrtel, suspicion grew that the Dane had perhaps deliberately halted visiting momentum and offered team-mates a breather at a time when, with Gerrard, Carragher and Downing finally on the pitch, Liverpool menaced.
Even so, he departed wearing a knee brace and O'Neill subsequently reported the "twisted" joint to be "sore and swollen", leaving the centre-forward a doubt for Saturday's FA Cup sixth-round tie at Everton when Sunderland's most consistent forward, Stéphane Sessègnon, will be serving the second game of a three-match suspension. "I'm desperately hoping Bendtner pulls through," said O'Neill, who will anxiously await the results of a scan on Monday. "He has enormous self belief – when Steve Bruce took him to Birmingham on loan from Arsenal as an 18-year-old he was trying to pick the team – and we have a few jokes at Nicklas's expense but he's strong and gives us presence."
Tearing down Sunderland's left, James McClean sporadically worried Liverpool but, in the first half at least, O'Neill's players looked equally nervous of Bellamy. The only shame was that too many team-mates proved unequal to his promptings, something horribly apparent by the time a clearly furious Bellamy was withdrawn, presumably to preserve his troublesome knees before Tuesday's Merseyside derby at Anfield.
In theory Suárez should have been on a similarly astute wavelength but the Uruguayan was repeatedly second-guessed by John O'Shea and Michael Turner when that increasingly formidable pair may have been more fazed by Downing's crosses and Carroll's penalty area physicality.
By the time the latter two were united in combat the game was lost but Dalglish refused to accept Liverpool's Champions League dream is over. "If we get fourth spot it would be fantastic," he said. But he is in danger of seeing his expensively assembled side ignominiously overtaken by a team like Sunderland or, much, much worse, Roy Hodgson's West Bromwich Albion.
Forget the idea of erasing Arsenal's 10-point advantage and somehow finishing fourth, Dalglish's real challenge is keeping Liverpool in the top seven.
Man of the match: John O'Shea (Sunderland)