After losing 1-0 to Newcastle in the Premier League last Sunday, Manchester United fell further behind rivals Manchester City in the table. 

Jose Mourinho's team are now 16 points off the leaders but still remain their closest rivals in second place.

Of course, with a squad like theirs, United were expected to brush the Toon aside at St James' Park, to keep the pressure on their city rivals. 

That didn't happen, thanks to a second-half strike from Matt Ritchie that secured all three points for the home side. 

No doubt, Mourinho will have already begun an inquest into what went wrong at the weekend, ahead of crucial FA Cup and Champions League games against Huddersfield Town and Sevilla this week. 

And according to club legend Roy Keane, there's one position he needs to pay close attention to as this season draws to a conclusion. 

"They haven't sorted out the defensive problems they have had over the last few years and that will continue," he said on ITV, per the Mirror. 

Despite all the attacking firepower Mourinho has added to his ranks over the last two seasons, the former United midfielder still thinks they're going to struggle unless the fix mistakes at the back. 

And Keane also singled out one player that he thinks causes the team to struggle whenever he features in the back line. 

"The two centre-halves need to do better. I've always thought with Ashley Young in your back four, you are going to struggle."

The Englishman has made 20 starts in the league this season and is usually preferred to Luke Shaw at left-back in big games.  

But after featuring in back-to-back away defeats, Young's place could now be under threat if Mourinho decides to give Shaw a runout in an important game.

And that's a possibility, given that United have Huddersfield and Sevilla before facing Chelsea at Old Trafford next Sunday, in a fixture that could see Mourinho's old club leapfrog him into second.