It depends on when and where this story is taking place.
John Keegan, in The Mask of Command, points out the type of warfare (thus the when and where) determines what, in context, makes a successful general in terms of how they behave on the battlefield. By the time of Napoleon, as you point out, heroically charging into the breach is not the way a general is expected to behave, but Wellington (who Keegan used as one of his examples), did put himself at risk because he was close enough to be put under fire. The difference between him and your lieutenant is that at no point did Wellington ever expect to close to direct combat. That wasn't his job, and his soldiers knew it; they appreciated the fact that he was out there risking getting shot, but they didn't begrudge him the fact he wasn't in the melee itself.
On the other hand, based on what we know, Genghis Khan preferred to stand back and observe the battle from the distance, but given the wide nature of Mongolian horse warfare, where you'd have units ranging fast and widely over the battlefield, this makes a great deal of sense.
But by and large, once you hit gunpowder, it's unlikely you're going to get that combination of distance command requires and closeness to the the action if your general is halfway competent, and even then, warfare is dominated by mass formations so one individual is unlikely to make a difference. Except in one type of situation: where everything, even for a moment, goes horribly wrong. And these types of things have happened.
The general is commanding and all of a sudden his command group is surprised by an enemy unit (usually cavalry), the result of which the general and his staff have to engage in close combat to get out of the situation. Typically they get enough warning they ride like hell for the closest friendly unit, but there have been times where they've had to fight off the attack.
So there's your way of having what you want; just for a brief moment, an unexpected appearance of enemy forces out of nowhere forces the general to fight for their own life, up close and personal, until things are stabilized. It's not about cooly commanding the army any more, it's about going steel-to-steel in close quarters, probably on horseback, until the attack is beaten off, nearby units can respond, or the fight takes them near friendlies who drive off the attackers. It's not going to happen very often, but the Napoleonic age is where it certainly could.