Relief for Mourinho as United send City packing
Mata the hero as Reds set up Hammers clash
Juan Mata gave Manchester United a timely shot in the arm as Jose Mourinho's men extended Manchester City's alarming winless streak to progress to the League Cup quarter-finals.
The news got better for United when the post-match draw pitted them at home to West Ham for a place in the semi-finals.
City were a far cry from the side that had at times lit up Old Trafford last month, but that was little surprise given the main protagonists were absent as the Spaniard plumped for nine changes.
Mourinho made a comparatively restrained four alterations to the side humbled at Stamford Bridge, although neither team was able to get a first-half shot on target as they struggled for attacking fluency.
Willy Caballero impressively tipped a Paul Pogba strike on the post when United first threatened after half-time, but the City goalkeeper was unable to prevent Mata sweeping home minutes later.
United held on to knock out the reigning champions and secure Mourinho just his fourth win against Guardiola, which will surely put a smile on the Portuguese's face after he spoke of his troubles living alone in a Manchester hotel.
Pre-match bemusement at those comments, and Henrikh Mkhitaryan's continued absence, were followed by an early let-off for the hosts.
An abysmal start cost United dearly at Chelsea and Kelechi Iheanacho had the chance to strike a similarly early blow, only to misdirect his header over after Jesus Navas darted past the recalled Luke Shaw.
Another of Mourinho's returning quartet was the subject of penalty appeals soon after Aleix Garcia went down under pressure from Michael Carrick - but referee Mike Dean was uninterested.
The official was soon involved and booked Iheanacho for clattering Daley Blind, despite the striker clearly slipping on the heavily-watered Old Trafford pitch.
Antonio Valencia was also shown a yellow card for a late challenge in a disjointed first half, with wayward passing and sturdy defences preventing either side from getting a shot on target. There was, though, an apparent injection of urgency from both sides as the break approached.
Zlatan Ibrahimovic had an effort blocked following neat work down the left, before the veteran Swede's fine pass put through Marcus Rashford, only for Pablo Maffeo to thwart the teenager.
Ibrahimovic and Pogba, so poor in the first half, combined and the latter got away a left-footed shot bound for the net until Caballero impressively tipped the ball onto the post.
Pogba tried another off balance and Mata's cross-shot stung the palms as the hosts pushed for a goal that arrived in the 54th minute.
Ibrahimovic stayed strong in a duel that left Nicolas Otamendi on the deck, then continued to send in a low cross which Ander Herrera missed as he clashed with Fernando. Referee Dean waved away calls for a foul and Mata swept home convincingly.
It was just reward for a markedly improved second-half display, with Ibrahimovic unable to get a foot on a fine Rashford cross and extend United's advantage. Sergio Aguero and Raheem Sterling were introduced in a bid to sway the tie, but it was to no avail.