Here's the entry for Fighting Style (Protection) from the PHB, pp 84: "When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. You must be wearing a shield."
I've been specifying a protectee because we're in a PbP environment and if ever Dent was to be surrounded by more than one ally, it's more efficient for me to specify in advance who Dent protects. I figured no one would want to wait on the DM holding up init to query me whether Dent protects ally x when ally x gets hit. Granted, my declaring the protectee limits who Dent protects that round (an adjacent ally could get hit but would be SOL if I specified a different person to protect), but I weighted efficiency over flexibility because of the slow nature of PbP.
The other thing about the grammar of the PHB entry is that it specifies the singular: "a creature," "a target,", and "the attack roll." Since other PHB entries for other abilities specify plural (e.g. "targets") or make use of the word "any," I figured the use of the singular here meant that I needed to be specific, and that the protection would apply to only one attack made on a protectee.
Well, that was my reasoning, anyway. I'm certainly open to hearing others' interpretation, particularly since my read is by far the most limiting of the ability.