I'm currently dealing with the effects of how I handled the same situation.
I brought in Danny Welbeck from Brighton as a Squad Player to act as 2nd option/sub covering Brewster and my AMR, similar skill level as Brewster but 32 year old at the start of the season compared to Brewster at 23. Unfortunately neither is doing well at the moment, Brewster got top goalscorer in the championship with 59, but 13 games into the premier league and he's put 9 away.
Strongly considering finding a replacement at midseason, especially since Brewster has already agreed to reduce his playing time from Important Player to Regular Starter as a result of his performances and hasn't improved in the 3 games since - the latest one I subbed him off at halftime with a 5.9rating.
Problem is, Brewster holds a lot of influence in the dressing room and I already have a player unsettled from selling Chris Basham in the summer.