Midfield targets have so far eluded Bruce since promotion from the Championship in May, and the 52-year-old admitted he has found it difficult to bring new players in.
And after seeing his side beaten by Birmingham City in a pre-season friendly on Saturday, Bruce confirmed he would be pursuing central reinforcements in the coming weeks, having already boosted his front line.
"We'll still maybe look at another couple to add to the squad," he said after their 2-1 defeat at St. Andrew’s. "It's proving very, very difficult, more difficult than it's ever been.
"That's what the challenge is, you've got to find them at the right price and see if they'll come to us.
"Midfield is the area that concerns me most now we've got (Yannick) Sagbo and (Danny) Graham in. If Sone Aluko stays fit we've got a group of strikers who are a little bit different.
"We're still a bit thin in the middle of the park and that's the area we'll be looking at."
Stoke's Wilson Palacios has been linked with the club, and although Bruce is a fan, he refused to specifically discuss the Honduran.
"It's wrong for me to talk about other people's players," he added. "I'm going to be linked with him (Palacios) because he's been one of mine."
"I don't know if Stoke are going to let him go. He's certainly a player I like but we'll see."
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