Stick with form especially good form, wait until his form starts dropping, whilst introducing the new guy slowly with plenty of training, thus avoiding injuries that often occur with new signings, especially ones that are rushed into the starting eleven mid season.
The guy is doing a great job for you dropping him will ruin his confidence and morale will drop off then when the new guy doesn't come upto scratch, your fu*ked.
If his form was bad this would be a completely different answer.