The Spanish midfielder is absent until August at the earliest following knee surgery and Moyes is looking for others to provide the quality in the middle of the park.
Pienaar's cross found the head of Louis Saha in the win over Boro and Moyes is hoping he can continue to provide the ammunition for the strikers.
Moyes told the website: "Steven is playing very well at the minute and his cross for Louis was very important.
"He is a player who always gives us a chance of scoring a goal and we need him and Leon Osman to maintain their standards in the next couple of months. Steven had a slow start to the season - you have to remember he missed a good bit of time after breaking his toe - and he picked up another injury shortly after that.
"But he is the one who, at the minute, is showing really good signs and we would hope he can continue in the same vein of good form."