What: Shares of HCP (DOC 0.88%) are plunging, falling by 18% as of 3 p.m. ET as the company issued weaker-than-expected guidance for 2016.

So what: HCP beat estimates in the fourth quarter of 2015, only to disappoint investors with a weak outlook for 2016.

The company guided that it expected to generate funds from operations (FFO) of $2.74-$2.80 per share in 2016, well below the consensus estimate of FFO of $3.17 per share.

HCP warned that HCR ManorCare continued to face challenges, which it expects will continue into 2016. The REIT noted that "reduced growth outlook for the broader post-acute/SNF industry indicates challenges to the improvement in HCRMC's financial performance over the next few years."

Its recently filed annual report discloses that the company generated 23% of its revenue from HCR ManorCare in 2015.

The company took an $817 million impairment charge in the fourth quarter, reflecting its expectations for weaker performance. In addition to industry-related challenges, HCR ManorCare is currently under investigation by the Department of Justice, a quagmire that's costing the company $1 million per month in defense costs.

Now what: HCR ManorCare has been a problem for HCP in the past, with the REIT having once already cut the rent it charges to the company. On the conference call, executives stopped short of suggesting that another rent cut was in the cards, instead saying that rather than comment on specific solutions, HCP would evaluate all solutions.

"Resolving the HCR ManorCare portfolio issue is our highest priority," said Lauralee Martin, HCP's president and CEO.