Gus Poyet has admitted he would have liked to have seen more of a new strike pairing at Hillsborough.
But he felt he was denied the opportunity by "scary" refereeing.
Craig Mackail-Smith and Leo Ulloa showed promise in attack after the Scottish international was sent on at half-time.
But Poyet opted to take Ulloa off 17 minutes later after he had been booked and subsequently spoken to again by referee Simon Hooper.
Asked if he would like to see more of the double act, Poyet said: “Of course but the referee didn’t let me play the last 30 minutes with those two together.
“It was coming that Leo was getting the second yellow card.
“It’s funny that you are a No.9 and you get fouled 30 times and you make one foul and you get a yellow.
“Then you make a second foul and the referee calls you apart. Scary really.
“It gets to a point where I need to change a player who is making a difference.
“We were smart to react quickly.”