Peter Andersson, the Washington Capitals' highly regarded rookie defenseman from Sweden, suffered a torn medial collateral ligament in the left knee during Sunday night's exhibition game with the Pittsburgh Penguins and could miss as much as half the season.

Dr. Carl MacCartee, team physician, diagnosed the tear yesterday. MacCartee will perform arthroscopic surgery on Andersson later this week, at which time the length of rehabilitation will be determined. The Capitals immediately recalled defenseman Paul MacKinnon from Hershey to replace Andersson.

MacKinnon had been assigned to the American League farm club on Sunday, hours before Andersson was cut down by former Capital Ted Bulley.

MacCartee's report on center Bob Carpenter, who suffered a dislocated right shoulder in the same game, was more favorable. Carpenter will be out from one to two weeks and a reevaluation Saturday will ascertain whether Carpenter can play in the regular-season opener at Philadelphia Oct. 6.

Carpenter, with five goals and six assists, ranks second to Alan Haworth in exhibition scoring. The Capitals are unbeaten with six victories and two ties. They will play the New York Rangers tonight at Richmond Coliseum at 7:30.

Coach Bryan Murray, who has shown evidence of a short fuse in previous incidents involving the ultraphysical and minitalented Penguins, was angry about the circumstances surrounding Andersson's injury.

"Bulley was running at guys all night and Peter tried to get out of the way, but Bulley caught him with a knee while his knee was planted," Murray said. "What really ticked me off was Bulley and Gary Rissling standing there laughing while Peter was lying on the ice.

"I told the guys, 'You stand around being pals with a guy like Bulley before the game, but he's trying to make that team and the only way he can do it is to run people.' "

Bulley was released by the Capitals in June. Although he was with Washington all last season, he scored only four goals in 39 games.

This was the second incident involving Andersson during an exhibition against Pittsburgh. In Wednesday's game against the Penguins at Hershey, Andersson was struck in the head by the stick of Pittsburgh's Pat Boutette, but was uninjured, since he was protected by both helmet and visor.

The big setback for Andersson is the loss of the opportunity to acclimate himself to the National Hockey League during the first half of the season. Most Europeans who come here need an adjustment period, because of the more physical play and the smaller ice surface.

Coincidentally, the man who now figures to carry the major load as the Capitals' fourth defenseman in Andersson's place is Timo Blomqvist, the Finn who experienced a similar problem when he first came here two years ago.

Blomqvist suffered a broken jaw during an exhibition game in Finland, missed a month and had so much difficulty as a result that he spent most of that season and part of the 1982-83 campaign commuting between Washington and Hershey.

Andersson was credited with five assists during six exhibitions. He had been a key figure in the Capitals' improved power play, which connected 12 times in 49 opportunities.

Carpenter's injury was a freak, as the shoulder popped out while he was winding up to throw a roundhouse right at Pittsburgh's Tim Hrynewich during yet one more battle in what has become a war between these mismatched teams. The shoulder went back in place when Carpenter fell, then loosened again as he rolled in agony.

Scott Stevens, the Capitals' policeman by default following Randy Holt's departure, barreled into Hrynewich and later was ejected for pounding Kevin McClelland after the Penguin center had used his stick to pull down goalie Al Jensen. Stevens has been thrown out three times in seven games, with a penalty total of 75 minutes.

"I'm not going to yell at Scott," Murray said. "I don't want to take away from his toughness and enthusiasm. Sure, we need him in the game and we've talked about the situation, but I don't really think he'll be challenged much once the season starts.

"I certainly couldn't fault him Sunday. McClelland cuts the legs out from under Al, right in front of the referee, and he doesn't do anything. Somebody has to do something, if we're going to rate any respect."