That's a tough situation... how to wake her up to a hard truth without discouraging her. But if she's serious about wanting to be really great, I guess the truth is your job here.
One possibility is to try to quantify some aspect(s) of performance (something that isn't so subjective she'd just argue them)... passes completed, passes received, defenders passed, possessions taken... something you could count from the sidelines and relevant to what she needs to improve. Then together you could compare her stats to some of the better players, so her belief about her performance isn't fantasy. Hopefully she can draw the necessary conclusion herself without you needing to say anything like "this shows you're actually not as good..."
You probably don't want to compare her against someone she can't possibly compete with, but someone who is just a little above her level, so she has some attainable goal to achieve. (And you may not want to name who you're comparing... it could be the average of a couple of better players or the unknown person playing her position on the other team) Then hopefully you and/or her coach can help her exercise those specific skills more until she succeeds. Then set another goal. Low frequency stats can be very jumpy from game to game, so if you're mathematically inclined at all it may be better to plot her average, and focus on raising her running average, not a specific per-game target.
You can use different stats and comparisons over time. When she gets to the point where she doesn't need the cold water of direct comparison and is willing to just work on improving stats, then you can stop making comparisons explicit and just give her stat goals to shoot for (behind the scenes you can determine what those numbers need to be).
It's also much harder for kids than adults to imagine how it's possible to improve if someone is better than you. You can't just tell a kid to "try harder" because that doesn't really work... if it's going to take an extra hour of passing practice a week, or training for more physical power, or some change of technique, then you and the coach have to be able to be quite specific about what she must do to improve a skill, and then make sure she succeeds, so that she learns it is possible. Sometimes kids will resist any talk of need for improvement because they simply don't see how to do it.