Depends on the player, but if it has a 5.1 output it almost certainly has a DD/DTS decoder, too. In that case, all you'll need is the 5.1 input. However, often the bass management & channel trims are more precise on the receiver, so you may want to consider using the digital input with DD/DTS. There are a lot of players with both coax & optical outputs.
Which model are you looking at? That would really help us to tell you if it has a decoder.