Now that I've seen Trafford start for us in 3 games (Palermo, Wolves, Spurs), there's enough to make preliminary assessments on his ability.
On the bright side:
Trafford is a very good passer of the ball.
Trafford is an excellent shot stopper.
Needs improvement:
Trafford's communication with his backfield needs a bit of improvement. At times defenders seem to expect one thing while Trafford expects another. I think that this is natural for a new keeper and will iron itself out over time.
Weaknesses:
Trafford often takes too much time on the ball. He's not ponderous by any means... but he's often closed down to the extent that his forward passes are deflected. IMO Trafford isn't used to the pace of the P/L - but as time goes on, I think he'll adjust.
Extremely poor decision making - in this category, Trafford's passing is most subject. He'll happily play a pass to a defender, facing him, completely unaware with an opponent literally millimeters away - with disastrous consequences - he's made such passes to Nico - leading to a goal - and in a different game to Reijnders who was aware of the pressure and immediately passed back.
Also in the extremely poor decision making category is that Trafford thinks that he has more time on the ball than he does. Numerous times Trafford is point blank with a defender with the ball at his feet.
Finally in the extremely poor decision making category is the times when Trafford leaves his box to charge forward - creating a wide open goal when staying in goal would have worked out much better.
Assessment:
Trafford may be the perfect keeper for us long term... or maybe not. He's an excellent passer and an excellent shot stopper. His range of passing is above average - but is no where near Ederson's.
If Trafford is our first choice in goal this season, we'll lose a lot of games due to his inexperience.
Conclusion:
Trafford has the potential to be a world class, first choice keeper. But he's not at all near this potential yet. I think that we should retain Trafford as our second choice keeper and keep Ederson if at all possible.
Moreover I think that the formula of Ederson always in the P/L and backup in most other ties - is completely misguided. Start your best keeper in all ties regardless of competition against strong sides - start your backup in all ties that you can afford to lose or against especially week opposition.
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TL/DR...
I think that Trafford has the potential to develop into a world class keeper, suitable for our system, which demands a keeper excellent on the ball.
That said, Trafford isn't yet nearly on this level - but as a #2 - is well worth keeping - as there's a good possibility that he'll develop from there.