Coutinho was just about to be introduced by Villa boss Gerrard midway through the second half when Bruno Fernandes slammed home his second goal of the game off the underside of the bar.
But the Brazilian proved the catalyst for a memorable response.
First he drove at the United defence, helping to create the space which Jacob Ramsey exploited to halve the deficit.
Then Ramsey slid over an excellent cross from the left which Raphael Varane was unable to intercept as he slid back, leaving Coutinho to finish into an empty net.
The goal brought celebrations at both ends of the pitch as Emiliano Martinez also went into the crowd in relief – it was the Argentine’s horrible sixth-minute error that had gifted Fernandes the visitors’ opener.
Coutinho the catalyst
Given his background – with Liverpool, Barcelona and Bayern Munich – Coutinho is a bold loan signing by Villa, and underlines the pedigree of the man they appointed as manager in November.
Gerrard is not at Villa to learn how to avoid relegation.
His demands remain as high as they were as a player and Coutinho’s arrival is an example of that.
Villa are a better team since Gerrard’s arrival and, after a sluggish start against opponents they dominated but lost to in the FA Cup five days ago, they grew into the game.
The club’s other newboy, Lucas Digne, was among those denied by United keeper David de Gea at the end of the first half – in unorthodox fashion, with his knees.
Ollie Watkins did not really test the Spain keeper when Villa broke after Edinson Cavani lost possession in his own half. However, Emi Buendia certainly did with a near-post header. Rooted to his line, De Gea’s reactions were up to the job as he beat Buendia’s effort away.
When De Gea produced more saves after half-time to deny Ramsey and Digne again, it appeared the image of the night for Villa would be midfielder Morgan Sanson kicking a drinks bottle in fury after his error in gifting Fred possession to set up Fernandes for the visitors’ second had been followed immediately by his substitution.
However, as it was the prelude to Coutinho’s arrival, it was the moment that changed the course of the game – and the acclaim for the Brazilian at the final whistle indicated Villa’s fans are already firmly behind him.
United display usual failings
This was another frustrating experience for United’s interim boss Ralf Rangnick who is trying, so far without success, to find a way for his team to get control of games and find the balance between defence and attack.
United could not have been given more of a gift to get their noses in front as Martinez appeared to be distracted by an off-the-ball run from Cavani and dropped what should have been a routine save from Fernandes 20-yard shot through his legs and into his net.
Yet, as was so often the case under predecessor Ole Gunnar Solskjaer, a bright start gave way to difficulties.
With Cristiano Ronaldo among the absentees, Rangnick went for a bold alternative in 19-year-old Swede Anthony Elanga.
The move worked to an extent but as United came under increasing pressure, their attacking threat was reduced to periodic counters.
Fernandes’ second – the first time he has scored more than one in a game since his opening-day hat-trick against Leeds – should have sealed victory. But they lacked the nous to see out the game and instead were pulled around far too easily.
After starting with back-to-back league wins, Rangnick has now seen his side muster five points from four games and they remain seventh, worryingly still a point behind a Tottenham side Antonio Conte is clearly not happy with, but who also have two games in hand on United.