Marcus Rashford's derby winner leaves City in a delicate position before Pep's arrival
Manchester United have put themselves right back in the conversation for a Top Four place and pretty much ended Manchester City's faint title hopes in one fell swoop.
United teenage sensation Marcus Rashford proved the hero for the Reds as he swivelled his way past Martin Demichelis and beat Joe Hart with a composed finish in the 1-0 win.
It leaves Man United sixth in the table, one point behind City who are clinging on to fourth place just above West Ham and Louis van Gaal's side.
They did have chances to equalise but Sergio Aguero and Wilfried Bony in particular both spurned those opportunities, with the Argentine coming closest when a near post header struck the post.
But they will rue a shoddy defensive display from Demichelis as well as injuries to Raheem Sterling and Joe Hart, which will add to their concerns as they look to secure Champions League football for next season before Pep Guardiola's summer arrival.
For now City remain fourth, but unless they pick up their form without injured leaders like Vincent Kompany and Hart, they are risking the unthinkable of missing out on next season's Champions League.
Manchester United's Marcus Rashford clashes with Manchester City's Fernandinho (left) and Martin Demichelis after claims for a Manchester United penalty during the Barclays Premier League match at the Etihad Stadium, Manchester.
Manuel Pellegrini is leaving and Guardiola is joining the club regardless of how the rest of the season pans out.
The Champions League could still offer some salvation by May, even if they are not among the favourites.
But missing out on next season's competition would be a huge blow for a club trying to establish a level of prestige by spending as much as the biggest clubs and bringing in the manager regarded as the best in the world.
The financial hit of missing out on one campaign in elite European football could be surmounted, with City's annual report for 2014/15 showing a small profit (by football standards) of £10.7 million. The club, like the rest of the Premier League, are also set to benefit from increased TV money as the new deal takes effect.
But the immediate effect on Guardiola's summer transfer plans would be more concerning.
Joe Hart is stretchered off with a calf injury sustained as he hastily cleared a short back-pass from Demichelis Picture by: Nigel French / PA Wire/Press Association Images
On one hand, the fact that Guardiola is manager could be considered attraction enough for potential new recruits to overlook the temporary lack of Champions League football.
But the very best players may just have that seed of doubt about joining the "project" - as modern managers and clubs like to call it these days - in order to either remain in the Champions League or join clubs of bigger stature and wealth who have qualified for next season's competition.
On the other hand, it would allow Guardiola to start building his team on three fronts (Premier League, FA Cup and League Cup) rather than four. But that's not what he, the players nor the fans would want due to the unrivalled prestige Champions League involvement provides.
As reports of Mourinho signing a pre-contract with United are doing the rounds, many are questioning what that means for young players' opportunities at Old Trafford.
To his credit, Louis van Gaal has not shied away from giving many academy players a chance in the first team with derby hero Marcus Rashford the most spectacularly successful.
Mourinho, though, may actually give Rashford a chance since he is proving adept at scoring at the very top level - raw though the 18-year-old striker remains.
But it's the next Marcus Rashford that resides in the academy ranks who should be worried if Mourinho is unable to shed his general tradition for favouring the finished article.