The Mount injury situation was unfortunate but the thinking at the time was: We want an #8 who can carry the ball up the pitch, counter press well and get back and defend on transition. We put Eriksen in that #8 role and sure he progressed the ball through passing more than running but it was clear that Ten Hag wanted a runner. He kept saying if we didn't sign FdJ for instance, we'll replace his skillset by using players already in the squad. The whole Zidane Iqbal situation in the friendlies when Ten Hag first joined was a preview of that. Mainoo was still something of an unknown quantity and I don't think we planned to rely on him as much as we actually did.
Anyway, I don't think Sancho would have done well if we put him straight into that Eriksen role but there was probably some tweaking that was possible to get him through the middle