Manchester United's defensive crisis seemingly got even worse after a 2-1 defeat at St. James' Park. Concerns are now growing over Noussair Mazraoui after video footage emerged showing the defender limping as he left the stadium.Michael Carrick’s impressive start as Manchester United boss hit a significant roadblock during a frustrating night in the north East. The Red Devils failed to capitalise on a numerical advantage after Newcastle were reduced to 10 men before the interval, eventually succumbing 2-1 to a late William Osula strike.The defeat was compounded by the sight of Mazraoui in clear physical distress during the closing stages. Having only just returned to the starting line-up for the first time in months, the Morocco international appeared to be struggling to overcome the fitness issues that have plagued him this season. Indeed, post-match footage captured the 26-year-old exiting the stadium with a heavy limp, sparking fears of a lengthy layoff. The injury to Mazraoui highlights a growing vulnerability in United's defensive depth. With the former Bayern Munich man forced off, Tyrell Malacia was handed a rare appearance, but the Dutchman looked understandably rusty in what was only his second outing of the season. In addition, United are still having to contend with the absences of defenders Matthijs de Ligt (back), Lisandro Martinez (calf) and Patrick Dorgu (hamstring).The United manager didn't hide his disappointment with the defeat and the missed opportunity to punish a 10-man Newcastle side. He emphasised that the squad came with high expectations after their recent strong performances."Yeah, disappointed obviously, bitterly disappointed. It hurts tonight. We definitely came here in good shape, looking to get something from the game, if not win the game, and the way it panned out is obviously very disappointing and there's no two ways about that," said Carrick, quoted by the club's official website."Football's football and there's things that happen in the game, sometimes go with you, sometimes go away from you but there are certain things we've got to do. Of course, we can be better, we can be an awful lot better. We've put ourselves in a good position, in a decent position with the performances and the results we've had, so yeah, tonight hurts."Add GOAL.com as a preferred source on Google to see more of our reportingThe Red Devils now have a 10-day break to assess Mazraoui's fitness. Medical staff will be working overtime at Carrington to determine the extent of the injury before their next Premier League match.United will return to action on Sunday, March 15, for a crucial clash against Aston Villa at Old Trafford, before traveling to Bournemouth the following week. Level on points with fourth-placed Villa, Carrick's side will be aiming to secure a return to the Champions League as the primary objective for the rest of the season.