SANTA CLARA, Calif. — The greatest passer in N.F.L. history will start for the San Francisco 49ers on Sunday.

Granted, if you measure the position by such hackneyed standards as Super Bowl victories or career yardage, you might give the nod to this quarterback’s former mentor in New England, Tom Brady. But the highest career passer rating ever achieved by an N.F.L. quarterback with 90 or more attempts belongs to Jimmy Garoppolo. His brief but stirring mop-up performance against the Seattle Seahawks (2 of 2 passing, 18 yards, 1 touchdown) last weekend raised his career mark to 110.1.

Small sample size? Sure. But it’s all we have to go on at the moment. Even 49ers General Manager John Lynch and Coach Kyle Shanahan have much to learn about Garoppolo, who has more nicknames (Jimmy GQ, Prince Aladdin, the Great Garoppolo, to name a few) than N.F.L. regular-season starts (two). Start No. 3 will be against the Bears in Chicago, about 30 miles from his suburban hometown, Rolling Meadows, Ill.

Fans of the 49ers have been abuzz since their team, which is 1-10 and in its third consecutive losing season, traded a high second-round pick to the Patriots for Garoppolo on Oct. 30. Tempering the enthusiasm is the checkered history of Brady’s understudies after they have left New England.