Off came Dimitar Berbatov - and none too happy he was either, having scored eight of United's previous 12 goals in the league - and on came Javier Hernandez.
Perhaps the home fans at the Hawthorns felt relieved to see the back of Berbatov, given the Bulgarian's recent emergence as a match winner.
But they would have been wiser to fear the worst, because Chicharito is fast becoming the trump card that could swing the title race in United's favor.
The Mexican has made only six league starts since joining from Guadalajara in the summer, with a further seven appearances such as the one at the Hawthorns coming off the bench.
But that has not stopped him scoring five goals, placing him joint second in the United ranks alongside Nani, who has made more starts than Hernandez has appearances.
His influence has not been limited to the league though - he has also won matches in the Champions League and League Cup.
Chicharito's first competitive goal for United came against Valencia in the Mestalla just five minutes from the end of a match that would be pivotal in his side claiming top spot in Champions League Group C. He had been on the field just eight minutes.
It was October before Hernandez made his first league start at Old Trafford, and it took him just five minutes to get on the scoresheet against West Brom, though it was all too easy for United who eventually squandered a 2-0 lead to draw 2-2.
His next league outing came at the Britannia Stadium, which was something of a proving ground - handle Stoke away, and he can handle anything the Premier League throws at him.
Hernandez responded in style, flicking in an outrageous header to give United the lead in the first half, then scoring with four minutes left to sneak a 2-1 advantage that clinched their first away victory of the league season.
Two days later, he stepped off the bench with nine minutes left of the League Cup tie with Wolves at Old Trafford with the score at 2-2, and struck the winner in the 90th minute.
Last month, he again came on as a substitute in the second half against Wigan, and had only been on the field 12 minutes when he scored United's second in a 2-0 win.
Take a bow | Hernandez celebrates yet another intervention from the bench
Then came his intervention at the Hawthorns, 14 minutes after replacing the disgruntled Berbatov, when he secured just their second away win of the league season from a game that could so easily have gone the other way (and arguably would have had West Brom been awarded a first-half penalty for Gary Neville's challenge on Graham Dorrans, and not squandered a second-half spot-kick that was awarded).
It is a handy knack of being in the right place at the right time when his side need him most that is building the fear factor around Hernandez. How long before opposition teams and fans roll their eyes - "oh no, here comes the supersub"? Perhaps it is happening already.
It can become a self-fulfilling prophecy - the more Hernandez affects the outcome of a game after coming on from the bench, the more he, his team-mates and, most importantly, the opposition will begin to expect it.
He would not be the first United player to carry such an influence - Ole Gunnar Solskjaer was a key part of six title-winning sides but made more than a third of his appearances as a substitute.
The Norwegian struck four goals in the last 12 minutes of an 8-1 victory over Nottingham Forest in February 1999.
That was just the warm-up - nothing will compare to appearing from the bench in the 81st minute of the Champions League final three months later and stabbing home the winner with virtually the last kick of the game.
Chicharito has a long way to go to emulate such feats - but he is well on his way to becoming the next great supersub.
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