The damage suffered meant he would not be able to cope with the high G-forces generated during a full Grand Prix lasting more than 90 minutes.
He said: "You can imagine that I feel frustrated and quite sad, because due to the circumstances [of Massa's accident] I wasn't exactly expecting to be ready.
"We have prepared ourselves very seriously, we have done as much as we could do from our side.
"But right from the beginning, Ferrari and ourselves always mentioned that I would do the job under the condition of being ready for it. It's sad that in the end I couldn't fulfil this target."
Massa's place alongside Kimi Raikkonen will now be taken by Ferrari test driver Luca Badoer. Even though the veteran Italian has never scored a point, nor raced since 1999, his call-up won Schumacher's support.
He said: "Luca has prepared himself quite strongly because this was his main job, to be ready in case something happened.
"Yes, he hasn't raced for a long time, but a racer doesn't lose the racing spirit and I wish him well."
Massa is expected to be out for the season after fracturing his skull in a freak accident during practice for the Hungarian Grand Prix last month.
Schumacher added: "We spoke yesterday and he is good and motivated to get back as soon as possible."
