A sometimes thrilling, sometimes nervous night for Manchester United ended with the home side in clear charge of this two-legged tie, but the comfortable margin of victory was belied by a performance filled with jittery defensive moments.
With Marouane Fellaini in to replace the unwell Henrikh Mkhitaryan, Jose Mourinho shifted the midfield shape around, but as a consequence United were appallingly open for the entire first half. Eric Bailly was at the centre of most of the visitors' chances, and he nearly conceded the opening goal with just two minutes gone, leaving a backpass two yards short of Sergio Romero.
United scrambled clear on that occasion, but the tone was set, and there was plenty more scrambling as the half went on. Bailly was left constantly exposed, partly by the ever-advanced Antonio Valencia and partly by his own peculiar decision-making, while Daley Blind was fortunate not to concede a penalty after sticking a foot out blindly. Maybe he was going for the pun. Certainly, when a defence looks to be lacking the calm, assured presence of Phil Jones, something funny's going on.
More wholesome humour came up the other end, as United took the lead. Zlatan Ibrahimovic threw himself to the floor to win a freekick just outside the area, picked himself up, and hammered a low shot into the knees of the wall. Unfortunately for the defender in question, the ball squeezed through his legs, wrongfooted the keeper in the process, and ambled over the line at glacial pace.
United could have had more: Juan Mata, released by Paul Pogba, planted a difficult, over the shoulder volley straight at the goalkeeper, and Anthony Martial, after dancing sixty yards down the pitch, couldn't quite find the finish. Maybe a couple more goals might have prolonged Fellaini's night; in their absence, the Belgian was hooked at half-time.
Whether it was the introduction of Jesse Lingard — a more obvious Mkhitaryan analogue — or a few choice words from Mourinho, United were tighter after the break. Saint-Etienne retreated into themselves a touch, and United should have had their second just after the hour. But Pogba, probably United's best player overall, passed up a beautiful chance to grab the second, planting a header from two yards out into the crossbar.
If United needed a reminder that one-goal leads are delicate things, they got it after 72 minutes. After some Martial-Pogba over-elaboration broke down in midfield, a quick exchange of passes left Nolan Roux free in the area. He succeeded in chipping Sergio Romero, but could only find the roof of the net.
Duly chastened, United went up the other end and doubled their lead. Marcus Rashford, on for Mata, took control of a loose ball on the edge of the penalty area, drove to the byline, and clipped a pass across the box. It found its way via the keeper's hand to Ibrahimovic, who poked into the untended net.
After that, there was only the questions of Ibrahimovic picking up the hat-trick and United clinging on to the clean sheet. The former came thanks to a rather generous penalty, won and converted by the big Swede. Quite how United managed the latter, nobody really knows.