Robbo's the go-to, even with his shaky moments. Cerny is definitely the #2 guy, and even goalkeepers get tired, so if we need to, we should rest Robbo against "weaker" opponents like Watford or the Spammers and save him for the matches where we're actually going to have a fight on our hands.
(This all being said, who could possibly NOT want a goal-scoring 'keeper as their #1?
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Alnwick has the potential there. We need to make sure he doesn't come up through the system quicker than he needs to, as Paul said, and the reasons for those aren't at all exclusive to the Beautiful Game. I've seen examples of it over and over again in ice hockey, where a player was rushed into the NHL and took much longer to get to their highest level of play. On the flip side, I've also seen examples of players who waited to make their top-level debut, then came in and dominated, in both football and ice hockey. So I say keep Alnwick in a position where he can refine his talent over time, then when he's really ready, start putting him in.