I was tech lead on a complex project for nearly 2 years and was eventually transitioned off of it after I got promoted. One of my senior coworkers ("Joe") was assigned to replace me on that project. Joe is one level higher than me in seniority, so in theory he would have been well qualified for this.
The transition period was pretty rough because it took Joe a while to learn the ropes. It took us nearly 3 times the amount of time that we expected it to take (our expectations were based on transitions with similar projects). Even once it was officially "transitioned", the project manager & myself both got frustrated after a while because it seems that Joe really just doesn't have what it takes.
I had acted as the architect and lead developer (there are junior developers involved too). Since the transition, the project has only been afloat because I keep extending myself to help (joining meetings, answering lots of very extensive questions, etc) but I know that I have to stop doing that eventually.
The project manager escalated concerns to our manager (Joe & I have the same manager). My manager scheduled time to discuss it with me.
I'm a drop concerned for that meeting because I don't know how to explain the situation in an objective way. Joe's generally pretty good at his job & I enjoy working with him; his brain just isn't wired for an architect role & the leadership step seems to be a bit too early for him.
Because of the seniority differences, I'm afraid that it will come across as arrogant from my end (ie. as if I'm saying: "I was so good that even a more senior colleague isn't able to take this on).
How can I phrase this in a way that's professional AND doesn't hurt Joe's reputation too much AND doesn't make me sound conceited?
Any specific phrases to use or to avoid?
If you were my manager, what type of details would you expect to hear from me, as "proof" that Joe lacks the needed skills?