I think he would either need to go learn his managerial trade elsewhere or for the club itself to be struggling financially or low in its league position for that to happen would be my take on things.
so I don't think you could ever rule it out happening.
a top notch player you cant take that from him - the fact he seems a quiet reserved type of lad from the outside looking in anyway - I don't see management in him - maybe a good No2 with I've no doubt a good footballing brain so who knows of course - he might have it in him to be a manager.
given his career he must have something to offer the youngsters at the club - so why wouldn't Boro use him in that area of the football club ..
Still, the best homegrown player to come out the club in the modern/riverside era - so that is a feather in his cap after all these years.
unless you are an out and, out success - you need thick skin to be a manager these days, especially with social media kicking about. Be interesting to see if it ever happens - it did for Mowbray and Woodgate - so you just never know with Steve Gibson in charge
https://www.gazettelive.co.uk/sport/football/football-news/stewart-downing-wants-complete-set-24124711