Manchester United have yet to keep a clean sheet this season and their back four has come under scrutiny

Manchester United's results under Michael Carrick have been enough to take the Red Devils back into the Champions League, but there are still some doubts. There have been questions asked at the back, with worries arguably exacerbated by a lack of summer investment.

The arrival of Carlos Baleba ought to help, with the Cameroon midfielder expected to provide protection for the back line when he's ready to make his debut after injury. However, the four men who started in defence at Everton on Sunday were also the four who began the painful 4-0 loss at Brentford some four years ago.

Then-manager Erik ten Hag responded to that result by breaking the bank for Casemiro, with the Brazilian thriving in his first season at Old Trafford before fading in his second. That back four of Diogo Dalot, Harry Maguire, Lisandro Martinez and Luke Shaw have combined for some decent results but questions remain over their ability to shut out opponents.

Carrick has used the combination in two games this season - against Ipswich and Everton - and they have played together 12 times in total. However, a return of four clean sheets in those 12 games isn't the best.

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In 12 matches with a back four of Dalot-Maguire-Martinez-Shaw, United have won eight, drawn two and lost two. On the surface, that doesn't look like too much to worry about, but the goals conceded column for those games reads 17.

Seven of those 12 encounters have ended with United's opponents scoring two or more goals. That includes two games this season and a trio of 3-2 wins last term, against Arsenal, Fulham and Nottingham Forest.

Spread over a whole season, 17 goals conceded in 12 games would equate to just under 54 goals conceded in 38 matches. Last season, only six teams in the league conceded more than 54 times and three of those were relegated.

If United want to improve on last season's third-place finish, they'll feel they need some improvement on that front. They have conceded 54 goals or more in just four of their Premier League seasons and their highest finish in any of those seasons is sixth.

On Sunday, United looked as though they had done enough for victory at Hill Dickinson Stadium when Benjamin Sesko struck late on. However, they were undone by a thunderbolt from Ainsley Maitland-Niles on his Everton debut.

"Of course, we don’t want to concede goals," Carrick told BBC Match of the Day. "Today was different from the others; it was about finishing the game off more than anything because we were in a really good position."

Baleba ought to help those behind him when he settles into his new role at Old Trafford. However, some may now find themselves wondering whether it's time to also make changes further back.