Baxter and Doyle were pressing marking other players, so I think it was ok for McCarthy to close down their target man. But when one member of the back four pushes up the remaining three have to drop deeper and pull together:
This is what we did:
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This is what we should have done:
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It would have left us with a back three instead of a back two, better equipped to deal with the danger. Fleetwood's wide man would have got the ball and would have had space to run into, but he wouldn't been able to pass it into space behind Flynn.
The full back position is new to Flynn, so not surprised he's making mistakes like these. It'll be up to the management and coaching staff to explain to him how he should be handling situations like these.
From the other angle, showing the run of the crosser:
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