West seemed (along with others ) to suffer form slumps after winning the League in 63 and 70 and was I think dropped and replaced by Rankin more times than he was replaced due to injury.
I also seem to remember Westy having a terrible form / confidence slump early in the 70-71 season, he was like Dai Davies on his worst day at the time. Especially at the ( 1st) Home game Vs Keflavik, the players were not happy with him and nor were the crowd, he had a bit of a go at them as well, it all fed on it's self in a downward spiral... result Harry dropped him
Short Version; Harry was never slow to drop a goalie who was playing poorly... 'big name' or 'first choice' made no difference. Stay in good form and don't get injured and you played...whoever you were.
Robles had done enough, IMO, to have got Harry's vote
google; 'andy rankin season by season' for the full picture.
Harry wouldn't have waited for Howard to get injured.
He would have dropped him after the Crystal Pakace home game.