Michael J. Boskin, President Bush's chief economic adviser, yesterday came close to saying the U.S. economy is entering a recession, acknowledging that "at best it is in a lull" that is likely to last until after mid-1991.

He said that after a long period of sustained growth, the economy is clearly in a slowdown that had begun even before the Aug. 2 Iraqi invasion of Kuwait.

The impact of higher oil prices would begin to be felt significantly in this quarter and the first three months of next year, Boskin said in a speech here to the American Council for Capital Formation.

Given the uncertainty about the outcome in the Persian Gulf, "we can't see how long or deep the oil shock will be," he said.

{In Tokyo early this morning, the currency and stock markets reacted to the new uncertainties about the situation in the gulf after Bush said he would send more troops to the region. The dollar was changing hands at 130.38 yen at late morning, up 0.88 yen from yesterday's close. The 225-issue Nikkei stock average, continuing a slide that began Tuesday, was down 1.51 percent.}

A new Bush administration economic forecast, to be completed by the Council of Economic Advisers next week, envisions the possibility of the economy "slipping into negative territory" in the current quarter, Boskin said.

Until now, Treasury Secretary Nicholas F. Brady and other administration officials have predicted that the economy, while sluggish, would maintain at least a small margin of growth.

In the first of two press conferences yesterday, Bush acknowledged that he is concerned about the slowing economy and will soon meet with his economic advisers to discuss a strategy for dealing with it.

One of the important items on his agenda, the president said, is lowering the capital gains tax -- a proposal that he pushed unsuccessfully during the recent deficit reduction negotiations.

The current administration economic projection, made Sept. 30, estimated that the fourth-quarter 1990 real growth over the fourth-quarter 1989 would be only 0.7 percent, a reduction from 2.2 percent estimated at midyear. The major reason for the change is the impact of higher fuel prices, which may cause a decline in the gross national product in the fourth quarter, Boskin said.

If the growth rate for the economy shifts from a positive 1.8 percent in the third quarter to a minus 0.5 percent in the fourth, the unemployment rate could increase from 5.7 percent in October to more than 6 percent in December and continue rising for the first few months of 1991, one economist said.

Boskin said that the "lull" would add about $30 billion to the budget deficit in fiscal 1991, presumably because of lost tax revenue and additional unemployment insurance costs.

While Boskin said that he was not predicting negative fourth-quarter growth, that would be the most likely time for the economy to turn downward. He added that the "lull" could last two to three quarters, overall.

The adviser not only avoided using the word "recession" -- which has negative political implications -- he made sure that his "lull" scenario did not match the accepted short-hand definition of a recession -- two consecutive quarters of negative economic growth.

He said the lull "will be brief and relatively mild... . The economy will rebound from the temporary hit" it is taking because of the Persian Gulf crisis and will recover once the problem is settled.

Then, he said, "the economy will pick up nicely. My best guess is that will occur sometime after the middle of next year."

Boskin said the current oil shock will be considerably less severe than the ones in 1973 and 1979, both because the United States has become more energy-efficient and because "we won't repeat the dumb mistakes" of the 1970s, including excessive regulation that he said created shortages.

"There are a lot of reasons for the economy to self-correct after the oil shock," he said.