Alex Ferguson "fears the worst" from Middlesbrough when they renew hostilities at Old Trafford on Saturday.
Boro have already had three acrimonious duels with Manchester United this season and are nursing a sense of injustice over Cristiano Ronaldo in particular.
Gareth Southgate branded Ronaldo a cheat for diving to win a crucial penalty during the first Premiership meeting at the Riverside earlier this season and a hotly-contested FA Cup sixth round tie, in which United emerged victorious after a replay, did nothing to ease the anger on Teesside at the Portugal winger.
James Morrison was sent off for hacking Ronaldo down late on, with skipper George Boateng following up by claiming the United man would "get hurt" one day if he carried on producing his party tricks against irate opponents.
With four wins from his side's final five games required to clinch the title, Ferguson has no intention of resting his star man. But the United boss is concerned at the tactics Boro may use to stop him.
"I am fearing the worst about what could happen," Ferguson said. "There is a paranoia about the boy and I think Middlesbrough will be aggressive tomorrow.
"We have important games coming up and I don't think they will be messing around."
Ferguson reiterated his concern at the FA's lack of response to Boateng's comments, which he believes have cast a shadow over the game.
"It was very provocative and I am surprised the FA didn't write to Boateng about it," he said. "I don't think Cristiano will be too bothered but it has set a different tone tomorrow, which could be a problem."
With a Champions League semi-final against AC Milan looming on Tuesday, Ferguson knows he cannot afford to be without Ronaldo, who is expected to be crowned PFA Player of the Year on Sunday.
"I have not thought about leaving Cristiano out," said Ferguson. "We have got to try to win the league and we need our best players on the pitch.
"I am sure he won't be affected by what has been said because the fantastic thing about him is that he is prepared to hold the ball at any given moment and wants to attack defenders.
"Then it is up to the referee to control the game and decide whether players are kicking him or fouling him."
