As worrying as that news should sound, given the quality of the player and the importance of the game to Manchester United, it would rather be a unsafe gamble to start the 24-year-old on Tuesday night - even if the medics certify him fit to play.
Lukaku pulled a goal back for the Red Devils with five minutes remaining but it proved to be nothing more than a consolation as Sevilla held on for the win to secure their spot in the last eight.
When Mourinho selects Fellaini ahead of the likes of Pogba or Mata, it says everything about the manager's approach and his reluctance to trust his flair players in a finely-balanced European fixture.
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The Spaniards progressed by the same score on aggregate after a goalless first-leg draw. We had risky situations in the first half, a good start in the second half and chances to score. It was that superiority in the middle of the pitch that helped Sevilla, now the fifth-place team in LaLiga, take down the favored Red Devils, now second in the Premier League. His goals earned the Spanish club a place in the quarterfinals and ensured that no opponent will under-estimate him in the next round.
That was before Sevilla shots flew high and wide of his goal for much of Tuesday night, a night on which United were sluggish and largely fortunate until De Gea could do nothing about Wissam Ben Yedder's fine finish.
Romelu Lukaku came close to scoring a goal with a strike in the 79th minute but he missed. Defensively, Sevilla have been leaky this season, allowed 42 goals in La Liga.
The official Sevilla expedition arrived in Manchester on Monday with the first group of 100 fans, who have steadily increasing with the arrival of more groups. Manchester United on the other hand could be licking their lips at this draw, and having gone to top notch form at Old Trafford, it could be as clear as day that Mourinho will win the tie.
"He did [train] after you left", Mourinho told the press.