Believing that a quality left-handed starting pitcher was the final component to ensure a National League West title this season, the Los Angeles Dodgers acquired veteran John Tudor Tuesday from the St. Louis Cardinals for veteran slugger Pedro Guerrero.

The trade, which was first seriously discussed over the weekend when Cardinals General Manager Dal Maxvill called Dodgers Executive Vice President Fred Claire, was completed Tuesday morning when Guerrero agreed to a contract extension for three seasons and $6.2 million, including a $400,000 bonus.Guerrero will receive $1.7 million next season, $1.95 million in 1990 and $2.15 million in 1991.

Tudor, 34, a veteran of nine seasons, gives the Dodgers a proven left-handed starter who helped pitch the Cardinals into the World Series in 1985 and 1987. Tudor, who missed most of spring training recovering from arthroscopic shoulder surgery, is 6-5 and has a 2.29 earned-run average, best in the National League.

Dodgers Manager Tom Lasorda, who will return to a five-man rotation, said Tudor was expected to fly to Los Angeles from St. Louis Wednesday morning and pitch Wednesday night against the Philadelphia Phillies.

Before the trade was made, Tudor was scheduled to pitch for the Cardinals Tuesday night.

To acquire Tudor, the Dodgers had to part with a proven slugger in Guerrero, 32, who has 171 home runs and 585 runs batted in a nine-season career with the Dodgers. But the injury-prone Guerrero, who had missed 52 days this season with a pinched nerve in his neck, was hitting .298 with five home runs and 35 RBI. The Dodgers's record without Guerrero in the lineup is 37-21. With Guerrero, they are 29-30.

Even given Guerrero's past contributions, Claire said he did not hesitate to make the trade because he felt that it was in the Dodgers's best interest. The team has been without a left-handed starter since July 31, when Fernando Valenzuela went on the disabled list with an injured shoulder. Valenzuela has not picked up a ball since then.

Just as the Cardinals risked losing Guerrero to free agency, had they not agreed to a contract extension before completing the trade, the Dodgers are at risk of losing Tudor after the season ends.

A clause in the contracts of all players with more than five year's of major league experience states that, if traded during the season, a player has the right to seek a trade two weeks after the conclusion of the World Series.

Tudor could seek a trade, naming six teams for which he would not like to play, and the Dodgers would have until March 15 to accommodate him or lose him to free agency.

Steve Fryer, Tudor's agent, said Tuesday that Tudor would be willing to waive the clause if the Dodgers provided monetary compensation.

But Claire said the Dodgers would deal with that aspect of Tudor's contract at the end of the season, when the contracts of many other Dodgers players expires.