Mike Dean steps up to referee the 2nd half of the Black Country Derby after Paul Tierney has torn his calf muscle
He only refereed Newcastle v Arsenal 24 hours ago
That is the massive risk in using someone as 4th official who has refereed the game before. Mike Dean has openly said that he can't referee both weekends and midweek any more as it takes him too long to recover, so I suspect he will be feeling it tomorrow after two games (or 1.5 games) inside 24 hours. Luckily it didn't cause a problem, but at some point it will.