AFTER 11 years as Harvard's defensive line coach and as John Yovicsin's top assistant, James S. Lentz is resigning to become head football coach at Bowdoin College. The announcement of his departure, like his work at Harvard, was made without fanfare. But his loss will be sorely felt.

As coordinator of the Crimson's defensive planning, Lentz has been largely responsible for the football team's consistent success on defense. In the last ten seasons Harvard has never finished below third place in Ivy League defensive standings. Since Yovicsin must devote much of his time to public relations and administrative chores, Lentz has filled a vacuum by providing enthusiasm and instruction to the team as a whole. Defensive players who have played professional football after graduating from Harvard have remarked that their training under Lentz was equal to, or better than, that of any big-time players.

Lentz, now 37, reached a point where he chose to build his own staff as a head coach rather than to remain an expert assistant for the rest of his life. His decision may bring glories to Bowdoin football, but it may also result in unfamiliar holes in Harvard's line next fall.