He was signed from Sunderland in January for £650,000.
Malky Mackay: "I think he is worth multi-millions. I thought when we got him we got him for a fantastic price.
“Fraizer Campbell showed against top, top defenders that if he... keeps playing like that there's a chance eventually he will be considered for England again”
"He was coming to the end of a contract [at Sunderland] and I thought it was one of those situations where you are in the right place at the right time for a player.
"I think he needed belief. He needed to be fitter and he needed the staff to put an arm round him and tell him what a talent he was.
"I firmly believe in him and I think he has got a lot of ability.
"He showed against top, top defenders that if he keeps his head down, keeps working and keeps playing like that there's a chance eventually he will be considered for England again."