Coach Joe Gibbs knew it might be difficult to replace Joe Theismann, but he probably never guessed how difficult it would turn out to be.

The Redskins are about two months away from the fifth anniversary of Theismann's final game and this week they've turned their starting quarterback position over to Stan Humphries for at least the next couple of months.

When the Redskins play at Phoenix on Sunday, it will be their 75th game since Lawrence Taylor's tackle broke Theismann's leg. The breakdown on Washington starters since that injury is: Jay Schroeder, 34 starts; Mark Rypien, 23 starts, and Doug Williams, 17 starts.

When Theismann got hurt, the Redskins thought they had a replacement ready in Schroeder, but that was only the beginning of the carousel.

Schroeder started the first 25 games after Theismann's injury. But he got hurt in the 1987 opener, and when he returned Oct. 17 (after a three-week strike), he was ineffective and eventually benched in Week 10.

Williams led the Redskins to a victory in Super Bowl XXII, Schroeder was traded, and since then, it had been Williams or Rypien until last February when Williams was released. At that point, it looked like Rypien was The Guy, but Gibbs made it clear Humphries now has a chance to establish himself as the No. 1 quarterback.

"You'd like to have someone step in and play 10 years as Joe did," Gibbs said. "We've had to work through a series of setbacks at quarterback, and Mark's injury is the latest. We've tried to work our way through it logically. It has been a constant series of obstacles. We're still looking long-term for the guy who's going to take us back to the Super Bowl. It's been a tough thing." E. Williams Questionable

Defensive lineman Eric Williams had a cortisone shot in his sore left shoulder Monday and said yesterday the pain in the area had subsided greatly. However, he probably won't practice today and is the only Redskin questionable for Sunday's game. Green 3 for 3

Cornerback Darrell Green has an interception in each of the first three games. He's tied for the NFC lead with Chicago's Lemuel Stinson. . . . The Redskins had been rotating their first seven offensive linemen, but against the Cowboys, the starting five played virtually the entire game. Raleigh McKenzie subbed for right guard Mark Schlereth briefly, and tackle Joe Jacoby played two series for Ed Simmons. . . .

Left tackle Jim Lachey is the only Redskin who has played every offensive down. . . . Kicker Chip Lohmiller leads the NFC in scoring with 27 points and is tied for the NFC lead in field goals with seven. He has kicked a field goal in 21 straight games. That's a Redskins record and places him third on the all-time list behind Fred Cox (31) and Jim Turner (28). . . .

Gerald Riggs has had just six carries the past two weeks after getting 13 against the Cardinals in the opener. . . . Art Monk caught only one pass on Sunday against the Cowboys, but it extended his streak to 103 straight games with at least one catch. During the streak, Monk has caught passes from Theismann, Schroeder, Williams and Rypien. . . .

The Redskins say their defense has played better than statistics say it has. Statistically, it's ranked 13th in the NFC, in front of only the Atlanta Falcons. The Redskins are 12th against the pass and sixth against the run. . . . The Redskins are ninth in total offense and are ranked 10th in rushing, fifth in passing.