He told Boro's website: "He will get better. He is starting to realise that he is an established player now but the age that he's at, there's still plenty of room for improvement and I feel we can set him a target to take him beyond what he achieved this season.
"When players have the quality Stewart has, it's inevitable there will be interest. Stewart knows what we're trying to achieve here and he's not for sale.
"I fought long and hard to keep him and we saw the impact on his performances he had after signing his new contract."