Rooney made history as the youngest-ever scorer at a European Championship on Thursday with two goals in England's 3-0 Group B win over Switzerland at Euro 2004.
But his volatile temper earned him a yellow card in Coimbra and Eriksson told a news conference on Friday: "If he gets a second yellow against Croatia, he couldn't play in the quarter-finals."
"It is a concern but you can't worry about that because we have to beat Croatia -- or get a draw -- or we can forget about the quarter-final."
"As a manager I can't think about leaving him out and hoping we can win the game anyway. He has to play against Croatia and that's it."
"Hopefully, he will be clever enough not to get a yellow card."
England have become heavily dependent on the 18-year-old with striker Michael Owen struggling to find a cutting edge and Rooney looking by far the more dangerous in Sunday's opening 2-1 defeat by France.
Eriksson was not taken aback by the euphoria surrounding Rooney after Thursday's match-winning performance.
"I'm not surprised," said the Swede. "He's 18-years-old, playing football the way he is and scoring two fantastic goals. He's the man of the day not only in England but in all of Europe."
Looking ahead, the former Serie A coach predicted a great future for the Liverpudlian, saying:"He's fantastic as he is but I think he will be even better in the future.
"I've had many young talents in the past, like Rui Costa, Roberto Baggio and Paulo Sousa, but Wayne Rooney is something very special I must say.
"He's not worried whether the opponents are France, Croatia, Switzerland or Turkey. It's as if he's going out on a school pitch playing football and saying 'give me the ball and I'll enjoy myself'."
Eriksson also moved to defend Owen, whose performances in Portugal have belied his impressive strike record for England of 25 goals in 58 games.
"I think he played better yesterday than he did against France," said Eriksson. "He was more involved in the game, dropping, taking the ball and making more runs on the left and right.
"I think he's getting there and I'm quite sure he will score goals. He hasn't done so far but he will."
Eriksson acknowledged England had played much better against France than in their nervy display against Switzerland.
"But maybe it's better to play badly and win, than play well and lose," he said.
Looking ahead to the Croatia game, he said: "We need better possession of the ball. We lost the ball many, many times in the first half in a bad way. But I'm sure we will be much better."