It's not really that it's a combination of L and R, it's that those two sounds are interchangeable in Japanese. It seems strange at first to native speakers of languages that use both sounds as distinct letters, but basically they are just two alternate pronunciations for the same letter and both are correct. The reason I imagine people mention D is because a short rolled R (like in Japanese or in Spanish as mentioned above) is often closer to a D than to an R in most English dialects. For example, if you say the word "ready" in an American accent, that D sound is already pretty close (if not indistinguishable in some cases) to a Japanese R. So if you have trouble rolling your R's, thinking of them as D's can be a good starting point.

It's very similar to the Spanish r if you're familiar with that.  English has a very unusual r compare to most languages.  It is sort of pronounced toward the back of the mouth and uses the whole tongue.  Instead many language use an R that is pronounced by tapping or flicking the roof of your mouth just behind your teeth with the tip of your tongue.  Have you ever tried making a rolled r?  For me once I mastered rolling r's, the short version like they use in Japanese became much easier for me to say.

Thanks for the reply! This really helped!