In my eyes, a subplot is a smaller story running through a bigger story, be that larger one a novel or film or TV series. They're good things, and I have plenty in my current work in progress. Where I'm coming a little aground is more of a plotette. My novel is, essentially, a tragic love story, and in it I have a situation where one of the characters is injured and there's much hair pulling and uncertainty about what's happening with her because they can't get in contact with one another. My problem lies in that once that injury is done with, it's done with and it feels like I shouldn't include an element that springs into the novel for only a few thousand words before being solved and letting the plot continue. I feel like it adds conflict, reveals character, all the things a good subplot should do, but it just isn't big enough for that label and I feel a little silly including it.
Any subplot works well when done right whether it be a thousand or five hundred words long. Don't use "being to short" as an excuse not to do it. If you feel it's too short just add onto it. There's a lot of things you can do with this situation to enhance and lengthen it given the scenario you described. Perhaps go into more detail about the incident and how it happened. How serious is the injury? Is it a life or death situation? Also another thing you could do is while this is happening give the hospitalized character some more backstory with flash backs. I don't know much about your story, so I can't truly understand where you may be finding issues. A good thing to do whenever you're having uncertainties about how the story is going is find ways to turn the issues you may have into ideas.
Tie it into future subplots, or even the main plot. Maybe something unpleasant happens that the injured person is unable to stop, or the lack of communication leads to one character taking an unnecessary risk.