Manchester United have now gone behind 17 times this season out of 30 matches. 10 times Van Persie has scored the winner or the equaliser in a match. His tally is now 20 goals in 21 starts and five substitute appearances.
By now, all those in the media and elsewhere who cast doubt on Sir Alex's wisdom in paying £22 million for a 29-year-old should be eating their words.
Roberto Mancini is choking and Arsene Wenger must be weeping.
But how good must it feel to know that even if you're losing a match you can bring on a star player who will get you a result? It's like playing your "Get Out of Jail Free" card in Monopoly.
And if United keep pulling results like this out of the fire, they could yet have a monopoly on the Premier League, the FA Cup and maybe the Champions League.
Too many poor performances
So, OK, maybe we tend to take the Scottish knight's words with a pinch of salt these days when he says he is taking the FA Cup seriously.
There have been too many times that he has blooded his youngsters and they've blown their own chance of getting good game time by crashing out early.
And guess what? One of the worst drubbings was at the hands of...West Ham.
On a wintry night in November 2010, Chris Smalling, Jonny Evans, Rafael da Silva, Ryan Giggs and Javier Hernandez got the bitter taste that United usually visit on their opponents, with a 4-0 drubbing.
Nobody does that anymore do they? (Well, there was that little local difficulty with the "noisy neighbours" last season.)
These days when United concede a lead or at least two goals, they almost always come back. And the beauty of the Cup is that Van Persie's 90th minute goal gets United a replay at Old Trafford.
You couldn't say West Ham deserved to win, but they didn't deserve to lose.
Manchester United definitely didn't deserve to win, and Sir Alex knows it. They dominated possession for much of the match, but their irritating arrogance in treating it like a training session nearly cost them. Too many players were below par.
And yet for the first six minutes United had well over 90 percent of the possession. So what? They didn't create a single chance until the ninth minute and only had 12 goal attempts in the whole match to West Ham's 15, despite 59 percent possession.
Of the team that started, Vidic, Smalling, Buttner, Rafael and Kagawa had a poor match and Valencia didn't add much when he came on.
The midfield needs fixing again Sir Alex
Michael Carrick was given the day off and didn't it show? Tom Cleverley was excellent again and has now found his goal touch.
But what does it take to get through to his manager that starting Paul Scholes is a recipe for disaster? OK, so the "Ginger Prince" didn't give the ball away and was stroking exquisite passes left, right and centre—but so what?
As is always the case when he plays, he slows the game down and plays too deep. Countless times Cleverley, Kagawa and even Hernandez had to come deep to get the ball. Eventually, and especially in the brief interlude between Scholes going off and Giggs coming on, the United midfield was overrun again.
A better team would have put the match out of sight. Take Tottenham. Scholes had great passing stats in that match, but United lost.
In fact Scholes or Giggs have started in central midfield in five of the six matches United have lost this season. In five of the games Scholes has started, United have been losing or drawing when he was substituted. Once again, he was withdrawn on a yellow card after several crude tackles.
And yet there is talk of Nick Powell going on loan to Peterborough.
Oh no, not the diamond...again?
Sir Alex tried the formation once more despite the blasting he got after the FC Braga match. It's not clear why he's doing it when he was recently lauding United's attacking football.
OK, so it worked against Newcastle in the Capital One Cup, but that was probably because Alan Pardew hadn't remotely expected it. And Cleverley, Anderson and Fletcher worked their socks off.
It is not at all clear why Sir Alex tried it again. The only key absence was Wayne Rooney, although Sir Alex chose to rest Carrick and keep Van Persie in reserve. Both his current first choice wide men were on the bench as well.
So, was the formation the manager's way of trying to accommodate Kagawa, Welbeck and Hernandez?
He may be still not quite match fit, but the Kagawa who looked quite sharp for 60 minutes against West Brom looked lost for much of this match. Sir Alex's perverse answer was to drop him into central midfield when Scholes went off, and that didn't work either.
So, both the manager and Shinji have a problem. Van Persie is an automatic choice and Hernandez and Welbeck are demanding starts. When Rooney comes back where does he play the Japanese wizard who looked so good at Dortmund?
So, eventually, the diamond was abandoned, but it was one moment of combined genius that saved face for Sir Alex.
Ryan Giggs, playing on the wing and told to run the West Ham defence ragged, hit a sublime early cross field ball 40 yards into the path of fellow substitute Van Persie. The Dutchman cooly slotted yet another world-class right foot goal.
So, what lessons were learned?
Those of us who thought Rafael could make a useful winger were confounded by an unusually mediocre performance.
Smalling was yet again playing in the young Brazilian's favourite position and demonstrating why he should never play there again: He looks clumsy and is easily overrun.
Meanwhile Captain Nemanja Vidic was confounding our recent relief at his return with a degree of uncertainty that we are not used to. He was directly to blame for both goals, as James Collins was left in acres of space to convert Joe Cole's inch-perfect crosses.
And maybe Alex Buttner needs more starts too because he was lost behind the diamond and didn't show his early-season promise until it was abandoned late in the match.
Sir Alex wasn't really convincing in his pre-match conference in wanting this one for Rio (who has never won a FA Cup winner's medal).
However, it is absolutely fair that he put out a team good enough to beat an average West Ham. The fact that they didn't was more down to United's shortcomings once again rather than the Hammers' endeavours.
And yet whatever team he picks for the return leg will probably run out winners. That is an extra match he could have done without, probably sandwiched between two tough matches against Liverpool and Spurs.
One thing is for sure: Robin Van Persie better stay fit, because he may be needed many more times this season to rescue his new teammates. Even if it is from the bench.