As mentioned above, the root ball needs to be opened up, if the tree has been in that pot for any length of time, the roots will have become fairly compacted, so they need to be freed up.
I have a little apple tree that came in a pot. My step-dad decided to replant it in the garden, but it never seemed to do very well, so last November I dug it up, which wasn’t difficult! All he’d done was pulled the tree out of the pot, scraped a bit of a hole and plonked the tree in it and left it. The poor thing’s roots were compressed into a solid ball, which took a fair bit of breaking up, and then I dug a much deeper hole, put compost in, put the tree in so the roots were well below ground level, then more compost and gave it lots of water. This spring it produced more flowers than it had for years, and lots of apples. They’re fairly small, but the tree needs a few years to really get itself established. They are delicious, though, very firm, juicy and sweet with a slight tartness. Really lovely to be able to pick and eat my own apples now.
Looking at that hole, I reckon it needs to be at least half as deep again, if not twice as deep, so that plenty of compost can be put in the bottom and around the roots. Lay it on it’s side, with to pot over the hole, get the pot off, break open the roots with a trowel or fork, then push it upright into the hole. Easy peasy. 😬