Clarke was pleased with the contribution of both Shane Long, who started, and Lukaku.
"They are a good combination. When they play together they play well but when I have my three midfield players in there it is difficult to play both," he said.
"Sometimes I have to make difficult decisions and in recent weeks Rom, for a 19-year-old, has been carrying a lot of weight up front.
"So it was nice to give him a breather and the chance to come into the game to do what he did.
"Rom did well but he got the benefit of the hard work of Shane Long, who put in a hell of a shift.
"He came on when the game was open - we knew Liverpool would come chasing the game and we could pick them off - and fortunately for the first time this year it worked."