Coaches in the National Football League are receiving letters from the commissioner, conveying his and the club presidents' concern over "the high potential for injury" of a chopping block on pass rushers.

The letters are not as forceful as those Pete Rozelle sent to the club president s before the opening of the 1977 season, when he deplored "unnecessary viclence" and threatened multigame suspensions of players whether or not infractions were detected by game officials.

A proposal to outlaw the chop block at players' knees did not come to a formal vote at a league meeting three weeks ago in New York because the club owners were widely divided.

Those in favor of a secondary block on a defensive lineman by a player coming out of the backfield on a pass play contended it is necessary to protect quarterbacks.

Those who opposed the "help" block cited the injury risk to the defensive end already occupied by a big offensive lineman.

Coach Tom Lanjry of the Dallas Cowboys suggested a last a temporary solution of the impasse by saying, "We are a group of professional people, why don't we coaches just agree not to teach that blocking technique?"

A consensus endorsed that idea, with the commissioner putting the authority of his office in favor of the pass rusher's safely.

Distinguishing from his threats of suspensions in his 1977 letter. Rozelle this time is expressing a subtle expectation of compliance about the chopblock. "Thank you in advance for your anticipated cooperation," he wrote.

The proposed rules change, which might he enacted later if that "cooperation" is not forthcoming, read:

"During a forward pass an offensive player aligned in a backfield position at the snap may not contact the outside pass rusher below the waist if such contact is made head-on or from an outside-in angle while the opponent is physically engaged by the blocking attempt of another offensive player."

In the Cowboys' game at New York last season running back Willie Spencer of the Giants applied a "help" block on defensive end Harvey Martin of the Cowboys, who was then thrown out of the game for kicking Spencen a league spokesman recalled.

Martin said afterward that he had injured knees and contended Spencer's block was well below the waist.

Alarm last season over a spine fracture that left wide receiver Darryl Stingley of New England largely paralyzed reinforced concern about player safety. Twenty safety measures have been adopted by the NFL in the last four years.

In 1977, only six starting quarterbacks finished the season without missing a game.

There were fewer quarterback injuries last season, but Tommy Kramer of Minnesota, Doug Williams of Tampa Bay, Bert Jones of Baltimore, Bob Griese of Miami, Roger Staubach of Dallas, Pat Haden of Los Angeles, Richard Todd of the New York Jets and Ken Anderson of Cincinnati were casualties.

Out of concern for their box office and television appeal, the club owners are ushering in the era of "soft touch football" for the quarterbacks this season with the narrower definition of a rule that figures to put game officials in more controversies than did the fumble calls of the last two years.

The officials are on order to be quicker in whistling a play dead when the quarterback is "firmly in the grasp or control of any tackler."

What if a strong quarterback such as Terry Bradshaw is grasped firmly by one hand but has his other hand free, with the ball in it? What if Staubach scrambles out of a tackler's grasp?

Nick Skorich, assistant supervisor of NFL officials, notes that in those cases the tacklers would not be ruled as having the passer "firmly in control."

Because the passer's body is largely exposed when he extends his arm above his head to release the ball, the NFL has outlawed the use of a helmet by a defender to butt, spear or ram a passer (or a receiver or runner) already in the grasp of a tackler. It will be interpreted as unnecessary roughness and draw a 15-yard penalty.