Rating agency Standard & Poor’s said it may cut its BBB rating of Ackman’s publicly traded Pershing Square Holdings vehicle in the wake of weak investment returns and a sharp drop in its net asset value.

“The fund’s performance since we initiated the rating on PSH in May 2015 has been weaker than peers, in contrast to its stellar track record in previous years,” S&P analysts wrote, noting that the fund lost money on 11 of 12 positions since October 2015.

A spokesman for the firm declined to comment.

Ackman’s Pershing Square oversees roughly $11 billion for wealthy clients, including state pension funds.

The fund’s net asset value tumbled to $3.8 billion this week from $5.3 billion in October 2015, the S&P analysts wrote. That pushed the fund’s debt-to-total-assets ratio to more than 20% from 15% at the end of October 2015.

The most immediate cause for Ackman’s woes is the sharp decline in drug company Valeant Pharmaceuticals’ stock price, which crashed more than 50% this week and cause Ackman’s firm to lose roughly $1 billion on paper. Valeant (VRX) delivered worse-than-expected financial guidance and raised the chance of defaulting on its debt if it breaches agreements with creditors by not filing its annual report on time.

This year alone, the Pershing Square Holdings fund, which has been trading publicly since 2014, lost 26.4 percent. That followed on a 20% drop in 2015. The year it was listed in Amsterdam, it scored a 40 percent gain.

As one of the hedge fund industry’s most widely followed investors, Ackman is now being vilified just as much as he was celebrated two years ago when he worked with Valeant to try to buy rival drug company Allergan, in which he owned shares.

Investors are privately saying they are concerned about Pershing Square‘s big bet on Valeant. Ackman has scrambled this week to soothe that concern by sending back-to-back memos urging clients to call him personally with questions.

S&P underscored his high-profile stature by writing “Bill Ackman’s prominent role exposes the fund to key man risk.”

But the analysts noted that Ackman has taken “proactive steps to respond to the turmoil.” On Wednesday the fund sold 20 million shares of snack food maker Mondelez (MDLZ), increasing its free cash. Last week Pershing Square Vice Chairman Steve Fraidin, a veteran mergers and acquisitions lawyer, joined Valeant’s board.