New job after 9 months in current one same employer

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What's the deal here. I joined a role last year in said company whereby one of the draws was the manager I was working under. He left recently to head up a new department in the same company but has expressed on a few instances what my career plans were and how good I was. Well roll on today I was called to his new office to discuss a work matter actually but it lead on to career again. I'm happy where I am to a certain extent and have not learnt the role conversely to say I'm at that time to move on. However this manager has substantial appraised me to the highest level in my mid year appraisal and having worked under him have had my attributes utilized to the fullest whereby I've been ranked highly on merit because of this.

Anyway I went back to my desk and messaged him saying I wanted to work in said dept.

He has said I need to let my current manager know.

However have I jumped the gun a bit as I have not received a formal offer from said manager just an interest in me. Should there not be an opportunity it may make things awkward in my current dept.