Those who expected Doug Rader, who as a player had a reputation as a prankster, to clown his way through his rookie season as manager of the Texas Rangers, apparently had never seen the other side of the man. ''Once the game starts, Rader is all business; as serious as any manager I've ever played for,'' said All-Star third baseman Buddy Bell of the Rangers. ''He's honest, he motivates well, and he's uncomplicated when it comes to letting you know what he wants. He's also good with kids because part of managing to him is teaching fundamentals, even at the big league level.''
Asked if Rader was the main reason Texas, which lost 98 games in 1982, leads the American League West by two games at the annual midseason All-Star break, Bell said you couldn't really single out any one person in the case of such a turnaround.
''When a team that has had a lot of problems suddenly begins winning regularly, I think you have to give most of the credit to the players,'' Bell pointed out. ''Sure, Doug has been great for this team, but we've had some people mature who were just learners last season. This isn't the same team I played on in 1982, either, in terms of overall personnel or attitude.''
The manager, of course, may have had something to do with this - as indicated by one of Bell's own experiences.
''Earlier this season, when I was in a horrible slump, Rader kept me in the lineup instead of sitting me down for awhile, the way a lot of managers probably would,'' Buddy said. ''I knew I'd bounce back and be OK again because I've been through these things before. But at the time I had no idea how much faith Doug had in my ability, so for a while that worried me more than not being able to hit. Well, he handled the situation perfectly as far as I'm concerned by letting me work it out myself.''
Getting Rader to talk about the tremendous job he has done so far is mostly an exercise in futility. Like Bell, Doug keeps saying that managers are only as good as their players. But I did learn that Rader spoke personally with every individual on the team during spring training, and that each was told exactly what was expected of him. Texas's strength so far has been its pitching and defense. Offensively the Rangers have hit well only in the clutch, which is nice timing, if you can get it. Fernando not as baffling this year
Although Fernando Valenzuela has stopped three Dodger losing streaks this season with his pitching, he obviously is not fooling National League hitters the way he did his first two years in Los Angeles. Among the things that Valenzuela has been struggling to overcome this season are early-inning control problems, plus throwing an occasional home run ball to batters who used to have trouble getting a loud foul against him.
''Opposing hitters eventually get to know what pitchers like to throw in certain situations, even pitchers the caliber of Fernando, and it takes away the element of surprise,'' explained Mike Brito, the scout who discovered Valenzuela. ''What happens is that the hitter, by being more patient and selective as the result of what he has learned, ends up swinging at more balls in the strike zone.
''Fernando, because he is smart, will eventually learn to cope with this situation by mixing his pitches better,'' Brito continued. ''But it is no longer realistic to expect him to go 15 or 20 innings at a stretch without giving up an earned run.'' As for Valenzuela's control problems, Mike says the Dodgers' first suggestion - that Fernando warm up longer before he pitches - hasn't worked so far. White Sox hot; Righetti tosses no-hitter
* Since Manager Tony LaRussa moved rookie first baseman Greg Walker into his starting lineup on June 12, the Chicago White Sox have won 15 games and lost only six. In his last 14 games, Walker has driven in 21 runs. Overall Greg has 37 RBIs in just 123 at-bats.
* New York left-hander Dave Righetti's 4-0 no-hitter against Boston on July 4 marked the first such occasion at Yankee Stadium since Don Larsen's perfect game victory over the Brooklyn Dodgers in the 1956 World Series. It was also the first no-hitter by a Yankee pitcher anywhere since then, and the first one tossed against the Red Sox since Tom Phoebus of the Orioles turned the trick in 1968 at Baltimore.
Righetti, in winning his 10th game of the year, threw 132 pitches while recording nine strikeouts.
* Manager Bob Lillis, whose Houston team lost its first nine games but has since climbed back to the .500 mark at this midway point of the season, says the Astros' current goal is not to lose what they have gained. ''For us to have a chance of winning our division, we can't afford any slumps. It's also imperative that we beat both the Dodgers and Braves almost every time we play them.''
* Hall of Famer Willie Mays was quoted recently as saying that Montreal Expos outfielder Andre Dawson is the best overall player in baseball. Dawson, a right-handed hitter with power, currently leads the National League in runs-batted-in, ranks third in home runs with 17, and is batting .321 in his bid for a fourth consecutive year as a .300 hitter.
* The Expos, incidentally, are pushing infielder Bryan Little for National League Rookie of the Year on the basis of his versatility plus his flair for bunting safely (10 so far), and coaxing even some of the league's best pitchers for walks.
* The Seattle Mariners, rather than go to salary arbitration with second baseman Julio Cruz at the end of the season, traded him to the Chicago White Sox recently for infielder Tony Bernazard. The Mariners were unhappy that Cruz didn't show more of a winning attitude, although he was aggressive on the bases. Meanwhile, the White Sox like the idea of speedster Rudy Law and Cruz batting 1- 2 in their lineup.
* From Kurt Bevacqua of the San Diego Padres on teammate Steve Garvey's fastidious nature: ''The reason Steve never gets his uniform dirty is because he always slides on the stripe that runs down the side of his pants.'' Garvey, the ex-Dodger first baseman who signed with the Padres as a free agent, seems headed for another 200-hit season. It would be the seventh of his career and his first since 1980.