Short answer: Yes. Long answer: maybe
The short answer is: yes. This should be possible. Macs have the ability to create so-called "one-wire" networks. Just connect any garden-variety ethernet cable from your NAS to your Mac and they will be mostly connected. If throughput is important, get a high-quality Category 6 (Cat6) cable, and don't make it longer than necessary.
Now for the "maybe" part. Since your two devices will be the only two things on the network, neither one will get an IP address assigned by something else in the network. Normally, your Mac or your NAS probably boots up and just makes a request into the network for a DHCP address. Something in your network (the router, most likely) assigns one. Your NAS and your Mac, however, probably act differently when nobody's out there to assign an address.
Your Mac will pick one from a special range of IP addresses. It will be 169.254.XXX.XXX. (Where the XXX.XXX part is just randomly picked). Your NAS may or may not do that. To make this work, you'll have to do one of two things: either change your Mac's IP address to be in the same network range as your NAS, or change the NAS's IP address to be in the same network as your Mac. Since I know MacOS and I don't know your NAS, I'll tell you what to do for the Mac.
Go into Settings, Network, and select your ethernet adapter. You should see a warning that you have a self-assigned IP address. That's normal. You need to know what IP setting your NAS uses. Once you know that, go into the network settings on your mac and tell it to configure the IPv4 address "manually" (it will say "DHCP" to start with). Then enter the numbers very similar (but not the same) as what your NAS uses. My recommendation: take the NAS's IP address and just add 1. If it's 192.168.0.199, set your Mac to be 192.168.0.200. Keep the netmask and everything else the same.
Without getting into how TCP/IP settings really work, that's the quickest advice I can give you. The main thing to know is that Macs automatically configure themselves for one-wire networks, and all you have to do is get compatible IP addresses to make a connection. I do this between my MacBook Pro and my old G4 tower all the time to get gigabit connectivity and do file transfers.