Manchester United manager Jose Mourinho has ruled star striker Romelu Lukaku out for a week after Lukaku suffered a scary head injury on Saturday against Southampton.

Lukaku was forced to exit the match after a head-to-head clash with Saints center back Wesley Hoedt. He was stretchered off and given an oxygen mask after being treated on the field for roughly six minutes.

Mourinho was asked about Lukaku’s condition prior to Monday’s match at Everton, and said, “I would say a week. It’s nothing really important – just a little protection, but no problem.”

It’s unclear exactly what Mourinho meant by “no problem,” and it’s slightly concerning that the Portuguese boss was so nonchalant about a head injury. It’s also unclear if “a week,” is an official timetable, or a minimum. And it’s unclear what the official diagnosis is.

Lukaku was, obviously, not in the matchday squad for the trip to Everton. A one-week absence would also see him miss United’s FA Cup tie with second-tier Derby County on Friday, Jan. 5. United’s next Premier League game is Jan. 15 against Stoke.

The Belgian forward sustained the injury when he jumped for an aerial challenge with Hoedt. Hoedt came charging out from the back, and tried to head the ball, but instead headed the back of Lukaku’s head.

Lukaku immediately reached for the back of his head, and lay prone on the ground. Multiple Manchester United players quickly signaled for the medical staff, which rushed onto the field. Physios held Lukaku’s head still while attending to him.

A stretcher was brought onto the field, and Lukaku was given an oxygen mask as he was lifted onto the stretcher and carried down the Old Trafford tunnel.

Marcus Rashford replaced Lukaku, and the game restarted after a six- or seven-minute delay.

Lukaku, who had sent an early header just over the crossbar, is United’s top scorer with 10 goals in the Premier League and 15 in all competitions. He had started all 21 league matches this season prior to the Everton game.

With Lukaku out, Mourinho will have to rely on Rashford, a 20-year-old forward who has spent much of the 2017-18 campaign playing on the left wing, and Anthony Martial. Martial started up top against Everton.

Mourinho said after Saturday’s match that Zlatan Ibrahimovic, the only natural Lukaku replacement, is out for a month with an injury.

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United and Southampton finished 0-0, and the Red Devils’ season is suddenly tumbling downhill.

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Henry Bushnell covers global soccer, and occasionally other ball games, for Yahoo Sports. Have a tip? Question? Comment? Email him at henrydbushnell@gmail.com or follow him on Twitter @HenryBushnell.