San Francisco 49ers rookie quarterback Trey Lance will miss a week after suffering a small fracture in his finger, head coach Kyle Shanahan said on Monday.

Though incumbent Jimmy Garoppolo has been the presumed starter throughout camp, Shanahan showed in Sunday's pre-season finale against the Las Vegas Raiders that he plans to use both of his QBs over the course of a game. 

In revealing that Lance was injured after hitting his throwing hand on another player's helmet, Shanahan did not sound concerned about how it would affect the big picture. 

"It wasn't bothering him much, because he just thought it was jammed on a helmet," Shanahan told reporters. "And then when you get all the images on it – when they showed it to me, it's a microchip of a bone that's off.

"He chipped it, which is a small chip fracture, which is worse than a jam. That's why it's not going to just heal on its own. He can't take snaps and do all that stuff right now.

"We're just going to rest it for seven days, they say, and that should make it heal and hopefully it'll be good next Monday when we get into practice."

Garoppolo and Lance rotated quarterback duties on the first two drives of the 34-10 win over the Raiders, with the veteran starting but Lance coming in after three plays for a change of pace. 

Lance completed six of 13 passes for 46 yards, while Garoppolo was four-of-seven passing for 64 yards. 

Neither quarterback had a touchdown pass or an interception, but each ran for a TD in the final tune-up as they previewed an unorthodox look that could give defences trouble. 

"I just wanted to get live action with it," Shanahan said of using both QBs on a single drive. "We've done it in practice a couple of times and this was the last time before we're going to get in the regular season. So it's the last time you can have the opportunity to do it. I kind of wanted to do it with this game."

The 49ers open the season at the Detroit Lions on September 12.