Griffin, who has been held out of preseason action, was cleared to play last week by Dr. James Andrews. However, Shanahan said Andrews still had concerns.
Shanahan spoke to Andrews to find out what those concerns were, but declined to provide details.
"You'll have to trust us," he said.
Andrews performed reconstructive knee surgery on Griffin in January, days after he was injured in a playoff game against the Seattle Seahawks.
ESPN reported last week that Andrews does not want the Redskins to expose Griffin's knee to excessive punishment during games. It's not clear which plays or type of plays concerned Andrews, but presumably it involves the read-option, which allows defenders to take a shot at Griffin.
The Redskins open against the Eagles on Sept. 9.
---Cincinnati Bengals defensive tackle Geno Atkins signed a five-year, $55 million contract extension.
Atkins, the son of former NFL safety Gene Atkins, can earn $31 million in the first three years of the new deal, which will also pay him $22 million guaranteed by March 2014.
Atkins, 25, was a fourth-round pick in 2010 and had 12.5 sacks in 2012. The two-time Pro Bowl pick has 23 career sacks despite starting just one game as a rookie.
---The Green Bay Packers signed quarterback Seneca Wallace, who is expected to serve as Aaron Rodgers' backup for the team's regular-season opener at San Francisco on Sunday.
Wallace left the 49ers last Thursday after spending one week with the team, although coach Jim Harbaugh did not confirm reports that Wallace told the team he intended to retire. What was clear was Wallace was not going to unseat Colt McCoy as Colin Kaepernick's backup in San Francisco.
The Packers released Vince Young on Saturday after hoping he would serve as a veteran backup. General manager Ted Thompson said he should have brought in Young sooner and gave him more time to learn the offense.
B.J. Coleman was released to create a roster spot for Wallace.
With the signing of Wallace, the Packers now have two quarterbacks who have insight into the 49ers' offense. Scott Tolzien, who is on Green Bay's practice squad, was the 49ers' No. 3 quarterback the past two seasons before being released last month.
---New York Giants wide receiver Victor Cruz and defensive end Jason Pierre-Paul were back at practice Monday.
Cruz was on the field for the first time since injuring his left heel Aug. 18 during a preseason loss to the Indianapolis Colts. Pierre-Paul returned from offseason back surgery.
Cruz participated in the no-pads practice and ran routes during individual drills. Coughlin said Pierre-Paul "has a ways to go."
---Brady Quinn was signed by the New York Jets, who continue to shuffle their quarterback depth chart leading into the regular season.
Graham Harrell was released to make room for Quinn, who lost the backup quarterback competition in Seattle to Tarvaris Jackson and was released by the Seahawks on Saturday.
Quinn is likely to serve as rookie Geno Smith's backup for Sunday's opener against the Tampa Bay Buccaneers with Mark Sanchez not expected to play due to a right shoulder injury.
---The Carolina Panthers signed free agent safety Quintin Mikell and placed safety Haruki Nakamura on injured reserve with a concussion.
Mikell spent the previous two seasons with the St. Louis Rams. He started all 32 games he played for the Rams and had 214 tackles, four sacks, two interceptions, seven passes defended and nine forced fumbles in that span.
For his career, Mikell has played in 155 regular season games with 91 starts and has 742 tackles, eight sacks, 12 interceptions.
---New England Patriots tight end Rob Gronkowski practiced for a second straight day Monday, but coach Bill Belichick did not say when he would be ready to play.