The 28-year-old is now pushing for a return in the Guinness Premiership when Bath arrive at Sixways Stadium next week.
Director of Rugby admits Horstmann's return - with Netani Talei ruled out for several weeks - will give the squad a timely boost.
And Olivier Sourgens, Dale Rasmussen and Graham Kitchener could all feature against Scarlets this Saturday in the cup (kick-off 3pm).
"We're hoping that Kai will play some part next week, it will depend on how he goes in training," said Ruddock.
"He is doing skills work and we are hoping he will take contact next week, if he does that then he might sneak into the panel for the Bath game.
"Even though Kai didn't have a lot of game time before we played Northampton on the first day, he basically came into the game from pre-season, and he scored a try and performed brilliantly.
"He is the sort of professional who can overcome difficulties like that and can come straight back into the team. We know if he comes back in he will be fantastic.
"Kai's return is good timing with the injury to Netani and we are waiting for the scans to come back on his shoulder.
"The knee looks like it will be a month, but the medical team is more concerned with the shoulder than they were after the game on Saturday. Netani hasn't slept well all week and they have sent him for scans, they are a little bit more fearful than they were before.
"Without Netani we have people like James Collins and Jake Abbott, who have experience in the first team.
"Matt Cox also played for Birmingham Solihull last week and had some experience in the Championship.
"We wait to see how Matt Cox will go, he will be in the panel this weekend and it is a great opportunity for him. He has been playing some great rugby for our A team, he was fantastic in our home game with Leicester Tigers A.
"If he does get a chance this weekend it is up to him to really put the pressure on me to cover Netani."