Pace is useful defensively, but it should not become the reason a player is always kept behind the ball.
At 9v9, a quick full-back or outside defender can be brilliant at carrying forward, overlapping, recovering and defending bigger spaces. That combination is probably more developmental than using the pace only as an emergency brake.
I’d look at what the player is actually learning. Are they being encouraged to step in with the ball? Can they join attacks? Do they experience higher positions in other games or training?
There is nothing wrong with a quick player becoming an excellent defender if they enjoy and understand the role.
The problem is when adults see an athletic quality and immediately build the position around what protects the team today.
Use the speed in both directions.
