So instead of pronounce, 'la le li lo lu', I pronounce it normally, which is, 'ra re ri ro ru'. Could the Japanese tell the difference?
If you simply cannot produce the Japanese sound, it'll work. I know someone who always used the English R, and it didn't sound as strange to my Japanese friends as it did to be as an American. R, L, and the Japanese consonant all sound similar to Japanese people, but not to me.
But, even if it sounds alright, you'll ultimately have a harder time speaking. Your mouth is in a completely different position, and R doesn't link vowels quite as distinctly as the actual Japanese consonant. You'll end up feeling like everything is a tongue-twister, and might accidentally substitute W for R, which will make your speech incomprehensible.
If you have to make a substitute, I think L is much closer. But I really advise practicing the Japanese sound.
For L, you just press your tongue against the roof of your mouth and hold it there. For Japanese らりるれろ, you flick your tongue a little bit behind that point, and don't relax your tongue quite as much.
You can practice by saying words like "butter" with a mock-English accent (butter->budda->badda->バラ). It sounds cheesy, but that's what did the trick for me seven years ago!