Chelsea have done this successfully for a while. He could theoretically be loaned out for another year or so.
I may sound silly, but given that I am not entirely sold on Depay just yet, and I have said many times that I would prefer a more direct dribbler like Moura or Costa - I'd probably choose Roberts. From what I've seen of him - he has a greater potential than Depay, and is the 'type' of winger I would prefer. Also, I can't help but to add his 'Englishness' as an extra credit in his favour.
I think with Di Maria and Mata as regulars, plus Januzaj and Young behind, we could get through another season with what we have. Roberts doesn't look like your regular 'promising player' too - he seems extra special, like a Raheem Sterling level talent. For an English player if this nature, I think you can justify taking a chance on him, they are so rare.