Pre-season had not even started and already a debate was ongoing about who might occupy the three centre-back spots for Manchester United's Premier League opener against Arsenal. Despite experiencing a torrid campaign last term, the season came to an end with United's centre-back pool in pretty good shape. Luke Shaw started four of the last five Premier League games, Leny Yoro returned for the Europa League final and Ayden Heaven was making rapid progress. As a result, there was no reason why United would have felt the need to sign a centre-back in this summer's transfer window. Instead, they have prioritised signings at the other end of the pitch this summer. Scoring goals was United's biggest problem last term, highlighted by their negative goal difference in the Premier League of -10. They will now be hoping the additions of Matheus Cunha , Bryan Mbeumo and Benjamin Sesko will solve that particular issue. While the aforementioned trio should be guaranteed starters for the Premier League curtain-raiser against the Gunners, there is less certainty about who will be starting in defence. Head coach Ruben Amorim's 3-4-2-1 system means three centre-back spots need filling, but Yoro is the only guaranteed starter. He started four of United's five pre-season friendlies and was excellent during the tour of the United States. He has nailed down the right-sided centre-back berth and will be one of the very first names on Sunday's teamsheet. However, it is not so easy to predict who will start in the middle of the back-three and on the left. That is because of the healthy level of competition Amorim has available to him right now. Harry Maguire and Matthijs de Ligt are leading contenders to start in the middle, while Shaw and Heaven are squabbling to start on the left. Maguire and Heaven got the nod against Fiorentina last weekend, but De Ligt and Shaw made more starts in pre-season, getting the nod on three occasions apiece. All four players will believe they are equally worthy of starting the season opener - and that is a luxury for any coach. Amorim needs strength in depth and competition for places. De Ligt experienced a mixed debut season at Old Trafford, meaning he will be looking for more consistency this term. Maguire revived his United career last term and will also be of the opinion he should start in the centre. Heaven , who will no doubt be desperate to start against his former club, has not put a foot wrong since joining United earlier this year and has developed a lot quicker than anyone could have anticipated. Shaw, meanwhile, will be out to make up for lost time after two injury-hit seasons. Amorim said of Shaws during pre-season: "I think Luke Shaw is a world-class player, that is my feeling. When he touches the ball, the ability to play one against one and you can feel in the small things. "England have reached two finals, and he had issues during the season but in the final he's always there. That means something, but he needs to be fit and train always in the limits. "I think he is doing that in the moment, so he is a great player, but he has competition this season, I am really pleased with Luke." Amid United's transition to a wing-back system, Shaw's best chance of becoming a regular at Old Trafford again is by playing at left-sided centre-back. He is excellent at progressing the ball from that area of the pitch to support United's build up play. However, his injury record is concerning to say the least. But with Lisandro Martinez also on the comeback trail, the England international should benefit from internal competition when he is fit. Such levels of competition will also be of comfort to Amorim, who will be hoping his players push each other to maintain high standards. The right-sided centre-back spot has been locked down by Yoro, but the other two positions remain up for grabs, and attempting to predict who will occupy them on Sunday afternoon is impossible. Only Sunday's teamsheet will provide the answers to this ongoing debate.