What: September was just an awful month for Memorial Production Partners LP (NASDAQ: MEMP) investors. Not only did the price of oil slide 8.5% last month, but the company was downgraded to a sell and tossed out of a popular MLP index. It's no wonder units were down 33.9%.

So what: It's a given that oil stocks like Memorial Production Partners move alongside the price of crude. So, with how steeply crude fell last month, it's not a shock to see Memorial's unit price get hammered. Unfortunately, however, that wasn't its only downside catalyst then. 

Friday September 11 was just a bad for the company. First, it was downgraded, along with several other MLPs, by Goldman Sachs from neutral to sell. Driving that downgrade is its view that oil will only average $45 per barrel next year, which is down from its previous view of $57 next year. That suggests that Memorial's cash flow next year will remain weak, putting more pressure on the company to further cut or even suspend its distribution.

In addition to that, the company found out that same day that it was being jettisoned from the Alerian MLP index. That index, which tracks the top 50 MLPs, tends to be matched by several funds that subsequently needed to dump Memorial Production Partners units.

Now what: The outlook for the oil market isn't very good right now with some suggesting that the price of oil won't recover next year. That would have an impact on Memorial Production Partners as it doesn't hedge all of its oil production, leaving that production exposed to those lower prices. Having said that, it is much less exposed to commodity prices overall because it does still have strong hedges, which suggests that when conditions improve, Memorial's unit price could rebound sharply.