Rain caused postponement of the Baltimore Orioles' game against the Boston Red Sox yesterday at Fenway Park. No makeup date was announced.

The clubs are scheduled to wind up the series at 2 p.m. today in Boston, but the Orioles will be without third baseman Doug DeCinces who, complaining of pain in his lower back, returned to Baltimore. He was told to rest and take heat treatments.

In Baltimore, Hank Peters, Oriole general manager, said DeCinces would be out for two games but would rejoined the club in Kansas City Friday. He said it was "too early to be pessimistic."

DeCinces complained of the problem Monday after the Orioles' 5-1 victory over the Red Sox. Back problems forced the slick-fielding DeCinces to miss 17 games last season, when he batted .249 in 145 games, with 16 home runs and 64 RBI.

In the first three games of 1981, DeCinces has had one hit in 11 at bats, a .091 average.

Should DeCinces be sidelined, Manager Earl Weaver is expected to move second baseman Rich Dauer to third and have utility man Lenn Sakata take over at second. Weaver had comtemplated that lineup change at the start of the season, when it was unclear whether DeCinces would recover from a cut heel.