Depending on whom you ask, Paul Pogba limping off might not be considered a frightening sight for Manchester United.

There were many who were quick to jump on the world's most expensive player's broad shoulders on Monday after he toiled against Chelsea and the veritable dynamo that is N'Golo Kante.

For Jose Mourinho, at least, it will be a major concern. Firstly, because he extolled Pogba's virtues to anyone who would listen in the wake of that FA Cup defeat. Secondly, because United's star power is waning at just the wrong time and it's making Champions League qualification far from certain.

Before Thursday's Europa League meeting with Rostov, Mourinho wrote in his programme notes in glowing terms about the loss against his old club. He hailed the efforts of his players and the fans as they battled for an hour with only 10 men, so soon after a long trip to face Rostov in the first leg of their last-16 tie on a pitch that has since been declared unfit for use by Russian authorities.

More pointedly, the United boss lauded his squad for coping with travel chaos – including an early-hours return from London on Tuesday – and the loss of players to injury and suspension.

"I've never once heard any of them complain," he said, perhaps because he's done all the complaining for them.

There was no lack of endeavour on Thursday, that's for sure. It's the quality on display that should have Mourinho on edge.

United did just about enough to reach the quarter-finals against a poor Rostov side, by virtue of Juan Mata's continued propensity to break the deadlock in tense home games. The 1-0 win at Old Trafford was otherwise a tepid affair, which they controlled for long spells but in which they failed to take charge on the scoreboard. What a tiresome sight that must be to home fans this season.

The extra worry – and it is a real one now – is that injuries are mounting. Anthony Martial and Wayne Rooney were not fit enough to make the squad, Pogba hobbled down the tunnel early in the second half with what looked like a hamstring problem, and Daley Blind's blow to the head saw Phil Jones introduced as a makeshift left wing-back.

Zlatan Ibrahimovic was playing again, but he'll be sidelined through suspension for the next two Premier League matches, as will Ander Herrera. That makes potentially six first-team players missing for the trip to Middlesbrough at the weekend. Failure to win at the Riverside Stadium would put a sizeable dent in their lingering top-four hopes.

The Europa League is looking more and more like United's best option to book that return to the Champions League. They will be favourites regardless of how the balls are drawn in Friday's ceremony in Nyon, especially given Mourinho's pedigree in continental competition.

But as the legs become that bit heavier in the final throes of the season and as the quality in this tournament becomes ever more refined, the need for a strong squad, and for key players to be fit and firing, will become paramount.

Pogba is one such pivotal figure, at least in the eyes of his manager. And with every injury United suffer, and every tame performance they produce, a Champions League spot will remain merely a tantalising vision on the horizon.