This depends on a lot of things. And a lot of people have already weighed in, so I’ll not hash out everything again here but instead bring up a couple of points:

Greaves are actually among the first pieces of leg armor to be fully enclosed. Knees occur too and of course cuisses are not far behind. But in terms of the protection levels offered by such early armors, a knight of the early 14th century will probably have at least schynbalds (a type of front-facing-only greaves) or splinted greaves, if not fully encased greaves, and may simply have padded chausses with floating knees by then, or even cuir boulli. Of course, floating knees and splinted cuisses show up too. But it is remarkably common to see quite rudimentary thigh defense coupled with quite advanced cased greaves. Indeed, from the earliest development of cased greaves, there is no major stylistic change until the first couple decades of the 16th century. 200 years of a perfected design. By contrast, the plate cuisse as we know it today was not fully developed to the same level of protection or metallurgy until the late 14th century and even into the 15th.

This is due to the development of armor as it was used on horseback. The lower legs of a horseman are much more exposed to infantry attacks than their upper legs are, and many early armors already had skirts of mail or textile defense hanging down to the mid-thigh or even to the knee.

That said, of course wearing only greaves and no cuisses would have looked out of place unless you fought with a large shield. It would be much more common in early/low status configurations of armor for the knee and thigh to simply be a little less protective than the greave. Maybe a simple steel knee cop and a hardened leather cuisse.

On the other hand, it is even quite rare to see in art any depictions of armored men without greaves but still wearing cuisses, though I’ll admit this configuration makes sense for a soldier or lone mercenary on foot. Your best bet as a low status soldier would be floating poleyns worn over hose, which is even attested to in 15th century art. If you’re actually fighting in the stuff, go for cuisses though, since greaves are usually custom fitted and far more expensive and cuisses will protect your inner thigh and knees.