Well, this is what I read into it:
"At this stage, the new Doctor had yet to be cast, but Dicks and producer Barry Letts -- who would himself be leaving Doctor Who at the end of the recording block -- were seriously contemplating casting an elderly actor in the role. They decided that this would necessitate the introduction of a new character who would be able to handle the physical aspects of the storylines, in the vein of Sixties companions such as Ian Chesterton and Jamie McCrimmon. Letts and Dicks therefore created a UNIT surgeon named Lieutenant Harry Sullivan (after possibly considering the surname Sweetman) to accompany the Doctor. Although UNIT was being gradually deemphasised, it was felt that Harry would also provide an enduring link to the organisation, thereby adding a familiar element to the new Doctor's adventures."
The article later points out that by the time Ian Marter had been contracted, they'd abandoned the notion of an older Doctor but kept the Harry character anyway.