This is how the Fifth Doctor started out - he was confused and rambling, talking to companions he'd left behind several regenerations before. At one point he thought he was the Second Doctor and Adric was Jamie.
Yup...ever since Pertwee, there’s been a sort of post-regeneration Trauma, worse in the case of Five and Eight because of the unusual regenerations, but it’s basically every time in the modern incarnations. Usually because of of odd circumstances, but also because it’s made a nice ‘old Doctor looking out of new Doctors eyes’ before the new guy settles in. It’s rare they just sit up...well, not least because everyone regenerates highlander style now...and inhabit the body.