With most tabletop RPGs there is going to be a certain level of disconnect between the mechanics of a mysterious effect and its, well, mysteriousness. Here are a few ways to handle this kind of situation sorted from most player knowledge to least.
- Describe the effect, explain the rules
This is probably the simplest option and the one I would recommend if you as DM feel that your plate is already pretty full. Start off with the flavor you want (feeling of unluck, feeling lethargic, things aren't where they should be, ...etc) and then describe to the player what mechanical effect this will have. This takes the burden of managing the curse off of you as DM and gives it to the player. The player, with the description given first, can then play up the effect based on what you describe and the results of future rolls. While it sounds like this would ruin the roleplay, players can frequently have fun describing how the curse affects them if they know what form it takes, even if they fully understand the mechanics out of character.
- Describe the effect, wait to explain rules
This is a slight varient of the above. Just like before, start with the roleplay description of the effect but don't tell them immediately what it did. Instead, wait until they first trigger the curse (in this case on their first combat roll afterward, or if you want to be particularly devious, on their first successful one). This means that you need to keep track of when the first trigger actually happens, but can add a fun sense of dread and unknowing to new curses found by the party as they don't know exactly what form they'll take until they strike and never know if there is another aspect waiting to bite then they haven't found yet
- Describe the effect, hide the rules
I wouldn't recommend this for any effect where it will come up frequently or if discovering what the curse does isn't important but another example is to only give the roleplay description of what is happening. When it comes time to handle the mechanics of the curse, do it discreetly behind a screen or the like and then describe what happens if appropriate. In the case of the roll two take the lowest, you could wait until they succeed and then roll quietly out of sight. If the roll you did misses then you can take the opportunity to tell them that they in fact miss and feel the effects of the curse taking hold. Most players will probably be able to work out roughly what is going on if the pattern is obvious enough but this maximizes uncertainty despite the relatively straight forward mechanic. The major downside is that this adds a lot of extra stuff for the DM to keep track of on top of everything else so like previously mentioned I would caution against it without a good (and short) reason for doing so.
With all of the above listed methods you can add to the mysteriousness just by having some factors that are less obvious that effect the curse. Methods 2 and 3 are a lot more difficult to figure out if it only triggers on an attack immediately after one that did damage, though again these sorts of triggers add to the already taxing job of DMing, so use sparingly. Likewise should you decide to forstall the knowledge of the curse entirely until it triggers for the first time, you can easily wait to describe the effect similarly to method 2, just make it clear that there is a specific and potentially knowable thing that is happening and not just strange DM whims.
In the end a lot just comes down to description and a touch of flair. When a miss happens due to the curse, if the player doesn't say anything, drop a quick line about how the curse or mysterious effect caused their sure hit to fumble or their carefully aimed strike to go awry. With even just a few words, you can turn the most straight forward of mechanical exchanges into an interesting narrative flow that stays within the game world.