Maguire because he's a constant in our defence which has been plagued with injuries and it's filled with players who are neither good on the ball nor possess the intelligence that is required of defenders nowadays. He rarely gets injured, he's strong in the air and he can pass the ball adequately. He's not perfect and his price tag will always be used as a stick to beat him with by some but we can build a well-functioning defence around him.
Pogba because he's the alpha and the omega in our build-up plays. Like Maguire, he's not perfect. He's overconfident which makes him dilly-dally with the ball in dangerous areas and despite being 27 to 28, his proclivity towards the extravagant still hasn't been toned down in favour of a more balanced game. But both his vision and his passing range are incredible and he's also very capable of carrying the ball through the lines. In a team that basically deploys three forwards who all thrive on open spaces and on 1v1s (preferably while running with the ball at defenders), Pogba's ability to quickly transition the ball to the final third is very important.
Shaw, despite his injury/fitness issues. Firstly, his underlaps (when Rashford hugs the sideline) are one of the few viable options we have from the second line of attack. Secondly, he can tuck in and operate as a natural CB either in a three-men defence or when we shift to three at the back during the build up (which allows both Matic and AWB to move forward and subsequently create pockets of space for Pogba and the RW easier). Lastly, he's the second player we have who can carry the ball through the lines.
Bruno because he always aware of what's happening around him and he's always willing to create situations for the forwards to do their thing by moving close to them to set them up with a neat one-two, by taking defenders away from them with his passing/shooting skills and creating spaces in behind. Without him, things can become static upfront because we have players who are much better at creating opportunities for themselves than for others and Bruno's presence balances that.
As things stand, Martial belongs in this category too. With Rashford's back problems and with Ighalo and James being the first players in the rotation for the attacking trident, Tony is the only one who can actually lead the line for us. More importantly, he's the best finisher the club has and in tight games he can make the difference between zero, one or three points. If we get Sancho and thus create the option of utilizing Greenwood upfront (without the quality on the right being dropped considerably), we'll put ourselves in a much better position but until then, an injury to Martial can prove once more to be destructive for our season.