Wide point spread shifts in the National Football League are most often caused by a mid-week injury to a starting quarterback. You can imagine that if Patrick Mahomes tweaked his ankle at Wednesday’s practice and was not going to be available for Sunday’s game at Los Angeles, the line on that game would shift four or five points.

In the preseason, drastic shifts in lines are most often caused by quarterbacks too … but not necessarily by injury. More often, in the summer, the quarterback prompted point spread swing is because one coach declares that his starting quarterback is going to see extended play while his opponent is still running out players as if the game’s final score is not a major concern.

Case in point this week is seen in the game at Levi’s Stadium where the San Francisco 49ers host the Los Angeles Chargers.

The opening line on this game had the 49ers favored by five points which from our vantage point appeared a big number. What the books must have handicapped before either coach involved in the skirmish announced their game plans was the Chargers would rest their starters in preparation for the regular season and the 49ers owned a natural home field advantage plus a little.

Then a quarterback injury shuffled the quarterback deck.

The man listed second on the 49ers depth chart at quarterback is Mac Jones, the one-time New England Patriots starter suffered a mild injury last week that will keep him out of action on Saturday. Carter Bradley is listed third on the 49ers depth chart but this week San Francisco also added Nate Sudfeld to their roster.

The 31-year-old Sudfeld was a sixth-round draft choice of the Washington Redskins in 2016 and has been on the 49ers roster in the past. In fact, he was beaten out for the third slot on the San Francisco depth chart in 2022 by a seventh-round draft choice, Brock Purdy. That was the same season starter Jimmy Garoppolo and Trey Lance were injured, and Purdy got his shot beginning in December and has never relinquished the starting role since.

With Jones sidelined you can be sure that 49ers Head Coach Kyle Shanahan is not going to risk injury to his starter for too long on Saturday night, which means the bulk of the San Francisco quarterbacking duties in this game will fall to Sudfeld and Bradley.

This looks a lot different than when the books first set the line on this game and expected after Purdy’s brief stint behind center Jones would see extended action.

Then we look at the Chargers side of the quarterback situation.

Yes, their franchise QB Justin Herbert will start and also see limited action according to head coach Jim Harbaugh. But his backup, Trey Lance, has looked good during his first season running the Harbaugh offense.

Wait.

Trey Lance?

Right. He is the quarterback the 49ers traded a boatload of draft picks in 2021 to acquire with a first-round draft choice. It didn’t work out for Lance in San Francisco, but the man the Niners took with the last pick, seventh round, in the following draft turned to 49ers gold … Brock Purdy.

Okay, lots of moving parts here.

Jones’ injury forces the signing of Sudfeld and will lead to less playing time for Purdy. The Chargers are coming to town under the direction of the one-time 49ers head coach and the quarterback they banked on four years ago and have since moved on from.

On Sunday, after Herbert’s cameo to start the game, we will, or should I say the 49ers will, get a full dose of the quarterback they thought was going to be their future and wasn’t. He let the 49ers down after being their number one choice and now he has a chance to put them down as their preseason opponent.

The line looked out of whack when it opened with San Francisco favored by five and now has shifted all the way to the Chargers being road favorites.

The shift, as I see it, is for good reason.

Qoxhi Picks: Los Angeles Chargers (-1) over San Francisco 49ers