I'm surprised but it makes sense when you think about it, assuming Sam wanted 3 goalkeepers:
Hart
Forster
Heaton
Obviously Forster has hurt himself in training, so the possible replacements would be Butland (injured), Pickford (injured), Foster (retired from internationals), Green (sh*te), Ruddy (old and likely to have retired by the World Cup)... I guess McCarthy was next in line.