Drew Stanton just can't seem to make it through an entire preseason healthy. In his rookie season, Stanton suffered a knee injury and was placed on the injured reserve. The injury probably wasn't serious enough that Stanton had to go on the IR, but that's what the Lions decided to do with him. Then, after playing well in the first couple of preseason games last year, Stanton suffered a freak injury on the last snap of practice, hitting his thumb on a helmet.
Yesterday we found out that Stanton had an MRI done on his left knee, and today we learned that he will have arthroscopic surgery on it (done by Dr. James Andrews). The injury will likely keep Stanton out for "several weeks," but the Lions aren't going to put him on the IR just yet, instead waiting to see what happens.
In the meantime, the Lions have signed quarterback Brooks Bollinger. Offensive guard Terrence Metcalf was released to make room for Bollinger, who will probably see significant playing time on Thursday despite only having a couple days to learn the playbook. Bollinger worked out for the Lions last month alongside a couple other free agent quarterbacks, likely just in case a situation like this happened. That is why it's good to be prepared.