Drama had unfolded in Manchester before either side had kicked a ball as speculation regarding England forward Chloe Kelly's future circulated - and she was not named in the City squad.
Manager Gareth Taylor told Sky Sports that Kelly had trained on Saturday but picked up an ankle injury afterwards, although it did little to quieten rumours the 27-year-old could be leaving the club this month.
Meanwhile, United manager Skinner said he wanted his side to show "more personality" and they certainly did that in the opening half an hour.
United caused City to panic, pressing them relentlessly and forcing mistakes as the hosts persisted on playing out from the back and falling into traps.
Celin Bizet played a key role, robbing Ouahabi of possession in the build-up to Toone's second goal after the England international had earlier dinked the ball over goalkeeper Keating from a clever Hinata Miyazawa flick.
Galton could barely believe her luck when she was able to poke the ball over the line from a few yards in the 21st minute and striker Terland was frustrated not to add to United's tally.
Missing key players Lauren Hemp, Khadija Shaw and Alex Greenwood through injury, and losing 3-0 at home to their rivals - it was a bleak picture for City.
However, the chaos had only just got started.
Miedema's header five minutes before the break gave City a lifeline and Knaak quickly followed it up in added time to make it 3-2. Suddenly, Taylor's side were back in the game.
As the half-time whistle went, momentum was with City and hope had been restored, although it did not last long.
Keating's clearance was blocked by Terland seconds after the restart and Toone was allowed to slide the ball into an empty net, leaving the England goalkeeper writhing in frustration at yet another error on her part.
Meanwhile, Toone slid on her knees in front of United's supporters and held her arms outstretched in celebration as they chanted her name.
"We managed to get the momentum back to get it 3-2 but then we started in a horrible fashion in the second half which gave all the momentum back to United again," said Taylor afterwards.
"Of course, conceding nine seconds straight into the half is really difficult. We worked hard to get the momentum back and we gave it straight away. We still had good opportunities to score."
It was a perfect return to WSL action for the 25-year-old Toone, having scored in United's 7-0 FA Cup win over West Brom following a two-month absence with a calf injury.
The United academy graduate had not scored against City prior to Sunday's showdown and although the hosts continued to create chances, they could not find a way through the defensive barrier in front of them.
It was a damaging result for City in the title race and they will be desperate to get their injured players back as quickly as possible to ensure their plight does not get worse.