"coach the team better" is the blanket term for what needs to happen. Since this is a discussion forum it would maybe be fun to spend more than 3 seconds thinking about what he is actually going to do to accomplish that?
Who will he play? what tactical changes will he make? Ragnick is first and foremost a good sports director. He's not a managerial winning machine like what Tuchel turned out to be.
For example I see that a LOT of posters want AWB dropped. The option we have is Diogo Dalot who was the pivotal roundabout for Villareals right flank attack at Old Trafford. Will he persist with AWB?
I think on a temporary basis he'd be a good option, he has a holistic approach and could definitely set in motion some of the changes needed at Old Trafford, both on and off the pitch. He's very influential and there's good reason for that.
I'm less interested in that though and more interested in your question of "If someone came in now what would they do?" because I think there's honestly a lot that could be changed, but also a lot of constraints that make making decisions quite challenging. So it's an interesting question to discuss.
Speaking solely about how I'd approach it, while there's a bounty of quality it doesn't always match up with what I think the best tactical approach is. I strongly believe in having fullbacks who can facilitate both possession and advancement of the ball, yet Telles is an unknown quality, Dalot has never impressed, AWB isn't particularly gifted in the areas I'd want from my FB and Shaw of last season would have been perfect but he's painfully out of form at the moment. Because of my commitment to playing a relatively high line it would make fitting in Maguire challenging (alongside his worrying form), Lindelof isn't particularly suited to it either. Varane is perfect for this but is currently struggling with injuries and new to the club, Bailly technically suits this style due to his physicality and brilliant recovery pace, but in this set up I'd be demanding a lot of my backline in possession and awareness and due to his erratic nature it could be pretty calamitous.
The midfield is a bit of a frankenstein hybrid, with the players as a collective not really suiting any one style nor really complimenting each other either. Matic 5 years ago would have been perfect for the holding midfielder role, but isn't physically up to the standard anymore. Fred and McTominay both inject energy but neither would suit the holding midfielder role, neither really fit the profile of what I'd want in my number 8s either. Pogba is wonderfully talented but has always been the wrong shaped peg, and Fernandes has been brilliant since joining but is also difficult to fit in to many shapes and styles. DVB is still an unknown quality and while he had a good loan spell it's been a long time since Lingard has performed at United.
The front line has a lot of riches with Ronaldo, Cavani, Greenwood, Rashford, Martial and Sancho able to be called upon, but once again you've got a mishmash of styles and they're not the most complimentary assemble. A lot of them want to play in the same position, Ronaldo is very productive but is 36, Sancho is new to the club and has struggled a little, Rashford's only just coming back from an injury layoff and Martial has been out of form for a long time now.
Adding the squad problems to confidence being at rock bottom and it's quite tricky when it comes to making decisions, having said all that though there's still many things that can be done, even without having the luxury of preseason or a bedding in period. One of the biggest and immediate things that I imagine many caretaker coaches would usher in (certainly Rangnick) would be a different pressing system, certainly one with more intensity and one that is more coordinated. We saw in the Liverpool game that Ole tried to initiate one, and it was an awful, both completely impotent at causing disruption to the opposition and completely exposing the backline. Ideally you'd want time to get it functioning at 100%, but plenty of managers have come into a job midseason and instantly applied a version of their press, and done so with success and without exposing the side defensively. I've seen comments that the current Manchester United squad is incapable of pressing but that's not true, there's certainly a few players who aren't massively suited to it but overall it's perfectly plausible that this could implement a press. I think it's important too, you'd have to go back to the 2015/2016 season to find a league winner that didn't employ a
Another would be use of the ball. Even if the person who comes in isn't fully committed to keeping the ball, if you want to compete with the top sides in the league over the season you need to ensure that you have a lot of the ball, both for attacking and defensive reasons. Someone like Klopp varies massively in his approach to the likes of Tuchel and Guardiola, but even so his Liverpool side have consistently been in the top 3 for possession since his arrival. It's important for both not keeping the pressure off the defense, but also making sure that the team is competent at creating chances while dominating the ball, instead of relying on transitions to create these goalscoring opportunities. You can still capitalise on both of course as the likes of Liverpool do, but the important thing for my side would be the ability to control the game while being able to consistently create opportunities.
These are just a couple of changes that could be made, as I've already written too much on what is a fairly pointless if fun discussion. I've obviously done it through the prism of a certain type of manager, but even someone coming in with a completely different approach still has a lot of options to them, and at this point sorting out the defensive line would probably be one of the biggest priorities.