Mullan had been tipped to push for England honours in the coming weeks after an impressive start to the season, but Ruddock admitted his young forward now faced a spell on the sidelines after suffering hamstring trouble.
"We have suffered some nasty injuries. It was an expensive night, not just in terms of not getting the result and not performing," said the Worcester Director of Rugby.
"In terms of the injuries, it is tough for us because we have Matt Mullan with a hamstring injury that looks quite serious.
"There is the potential that it could ripped away from the bone and the attachment, I'm hoping that is not the case but it looks a serious hamstring injury. At minimum, I would have thought we are looking at six weeks.
"Until we get the result of a scan on it, it is difficult to say the timescales are. But the medics are usual pretty spot on and they say it is serious.
"We will know more after the scan, but I would say he was a major worry for any autumn internationals."
Mullan now joins a busy treatment table with Dale Rasmussen, Marcel Garvey, Shaun Ruwers and Graham Kitchener also forced off against the French side.
"Marcel has done his hand and could have a fracture, we have sent him off for an X-ray but that is a major worry for us," added Ruddock.
"Dale has done a grade one tear of the medial ligament in his knee, it' not the knee that he had taped up, it is the other one.
"Graham got dropped out of a line out and landed on his shoulder so his AC joint is very sore, to say the least. There is a huge worry there and we will get that checked out.
"Shaun ripped off a tendon on the bottom of his toe; that looks like a 12 week layoff initially. A freak accident and I don't think our medical boys have seen one of those on a forward.
"All those things disrupt us, but we are not going to hide behind that as an excuse. We just didn't perform as we could have done.
"We went away from our game plan and let them feed off our mistakes, we never really put them under pressure to ask the questions of them we wanted to."