The initial prognosis was that Bobb would only be out for 3-4 months, meaning that the club anticipated him returning at the end of December, before the winter window. It was only his slow return and subsequent injuries that lengthened his absence.
I can only imagine that the club, anticipating his early return, thought it had sufficient quality in place not to need to sign someone else, and nobody could have imagined both Grealish and Foden would have such collapses in form. I also suspect that the club, post Palmer, might have been a little stung and wanted to keep space for his return to show that it had academy talent on the conveyor belt. Just my musing.