Head Coach Hill later confirmed that Horstmann was set for a period of four to six weeks out and joins fellow back row James Freeman, who underwent surgery on his injured knee this week, on the sidelines.
He said: "Kai has pulled a hamstring and that doesn't look too good. That could be a four to six week layoff.
"It he has pulled it quite badly, by the looks of it, so on Monday night we will look at Adam Balding and the new signing Richard Muagututia.
"It is up to one of those two to step up and show us what they can do and demonstrate they are a worthy replacement for Friday night."
Hill is also set to the look at the Worcester lineout after it misfired in defeat to Harlequins on Saturday afternoon.
"The lineout did not function very well," he admitted. "We will have to look at it, some of our systems and calling wasn't good.
"That was a disappointment because Chris Jones can do that, he has been very good in training and he is an international player. But our lineout did not function without Craig Gillies starting."