Frank Johnson, whose four-point play at the end of a second overtime Thursday night enabled the Washington Bullets to beat the Detroit Pistons, 128-126, learned yesterday he will be sidelined six weeks.

Clearly, the celebration for the reserve guard was short-lived. "That was the good news," he told well-wishers. "Now let me tell you the bad news: I cracked a bone in my left foot."

He was not hurt on the dramatic 26-foot shot that gave the Bullets a 127-126 lead with seven seconds left in the second overtime. It happened on Washington's next possession, and no one else seemed to realize it.

With six seconds left, Detroit's John Long was called for walking. The Bullets inbounded the ball, and Johnson broke away against full-court pressure for a layup, chased by Brook Steppe.

To keep Steppe from fouling him, Johnson took an extra-long stride and tried to plant the foot for a layup. "The long stride probably did it," Johnson said. Sensing that he had cracked a bone, he kept dribbling under the basket and reversed direction toward the foul line, when Steppe fouled him.

"I told Gus (Williams), 'I think I broke my foot,' " he recalled yesterday. "He said, 'What?' "

Television replays show Johnson grimaced as he missed the first foul shot, then made the second for the final margin. He left Capital Centre to be X-rayed and learned he had cracked the fourth metatarsal of the left foot.

"It's unbelievable," he said yesterday. "I say it was sweet-bitter, instead of bitter-sweet."

Johnson, who underwent surgery in college for other broken bones in the same foot, is on crutches.

He joins Jeff Ruland (tendinitis in the right shoulder) and Cliff Robinson (sore right leg, knee and ankle) on the Bullets' injured list, although Robinson, who has missed 15 games, is to practice Monday.

Bullets Coach Gene Shue called the game one of the most incredible in which he had been involved. The Bullets made four three-point goals in the final 2:10 of regulation to tie the game; Detroit's Isiah Thomas (25 points, 24 assists, 10 rebounds) made a three-pointer to send the game into double overtime, and the Bullets rallied from five down with 1:51 to play in the second overtime.

When Thomas scooped in a layup with 17 seconds to play in the second overtime, Washington used its last timeout.

The Pistons had a foul to give, and Coach Chuck Daly told game officials Jim Capers and Bill Oakes his team would let the Bullets run some time off the clock, foul Williams around midcourt and make the Bullets pass inbounds again without another timeout to set up a play. From the Bullets' side of midcourt, Johnson inbounded to Williams, who dribbled toward midcourt, where Steppe tried to foul him.

"I thought I did a pretty good job of it," Steppe said. "I had one arm around his waist, and my knee was stuck way up his behind."

But there was no whistle. Not hearing one, Thomas left Johnson, whom he was guarding, to try to foul Williams, too. Johnson drifted down the left sideline. Williams waited until Thomas got out of his passing lane, then passed to Johnson, who dribbled and squared to shoot. He was open, and so was Jeff Malone in the corner. As Johnson began his shot, center Bill Laimbeer came off Tom McMillen, his man, to try to block the shot.

Instead, he fouled Johnson, whose three-pointer and foul shot was the 21st four-point play since the league adopted three-point shots in 1979.

"I saw Laimbeer coming. . ." Johnson said. "I already had gotten a nice, smooth follow-through . . . He pushed my hand back."

On the telephone, Frank Johnson's brother Eddie, a guard for the Atlanta Hawks who had watched the cable telecast, said: "I saw when it left your hand. I knew it was good."