The Texas Rangers fired Manager Doug Rader yesterday and replaced him with Bobby Valentine, the New York Mets' third base coach.

Valentine, who has not managed in the major or minor leagues, was reported to have agreed to a three-year contract following a meeting in Houston with Eddie Chiles, the Rangers' chairman of the board; President Mike Stone, and General Manager Tom Grieve.

At 35, Valentine becomes the youngest manager in the big leagues. He will take over the Rangers in Chicago Friday night for the start of a three-game series with the White Sox.

Rader's contract with the Rangers will run through 1987. Texas hired him in November 1983 as their 11th manager in 12 years. Rumors of his dismissal have been circulating the past few days.

"What can I say?" Rader said with a shrug. "I certainly can't say anything original, anything all the others didn't say. I'm very glad to have been given the opportunity."

Valentine inherits a team with a 9-23 record, the worst in the majors. The Rangers lost their sixth straight yesterday, to the Yankees, and are 10 1/2 games out of first in the American League's West Division. Knuckleballer Charlie Hough is the only member of the pitching staff with a complete game.

Under Rader, the Rangers finished third in their division in 1983 and seventh last year.

Yesterday at Yankee Stadium, Dave Winfield's bases-loaded grounder scored pinch runner Rickey Henderson with two out in the ninth inning to give the Yankees a 6-5 victory over the Rangers.

Pinch hitter Ron Hassey led off the ninth with a single past second baseman Wayne Tolleson. Henderson ran for Hassey and took second on a balk by Dave Stewart (0-3). After Bobby Meacham sacrificed Henderson to third, Willie Randolph and Don Mattingly were intentionally walked to load the bases.

Winfield hit a grounder to shortstop Curtis Wilkerson, but the Rangers could get only the out at second as Henderson scored.

Dave Righetti pitched one-third of an inning for New York to raise his record to 3-2.

New York tied the score at 5 in the eighth. Don Baylor hit his sixth homer of the season, leading off against Stewart.

The Rangers took a 5-4 lead with a run in the seventh. Tolleson led off with a single and stole second and third. With one out, Buddy Bell hit his third double of the game, which went under the glove of left fielder Ken Griffey.