SANTA CLARA, Calif. -- If the Detroit Lions are without Darius Slay for next week's prime-time showdown with Tom Brady and the New England Patriots, Teez Tabor is expected to get the start at cornerback.

If his first start is any indication, that could be a problem.

Tabor was inserted into the starting lineup over Nevin Lawson on Sunday, but was benched after committing crucial mistakes that led to two scores on San Francisco's first three drives in the 30-27 loss.

Not exactly the kind of stuff that inspires confidence heading into a matchup with the GOAT, especially with Slay now sidelined by a concussion.

"I mean, I have confidence in all our players," coach Matt Patricia said. "That's why they're here, to go out and play well. I think Tabor does a good job every week of being ready to go. He studies, he's smart. He's a big, long guy that can go out there and obviously have a good physical presence. I'm always confident that he's ready to go."

But on Sunday, he didn't look ready at all.

On the defense's first play of the game, Tabor was dragged about 15 yards downfield by running back Matt Breida. Two plays later, he bailed out a broken play with an illegal contact penalty. One play later, he was beat for a 35-yard pass.

San Francisco was picking on him. And it worked. It suddenly had the ball in the red zone, and wound up getting three points out of it.

Then two drives later, Tabor chose a bad time to play man while the rest of his defense played zone. The 49ers were sitting on Detroit's 4-yard line. Tabor cut inside with his man, and Jimmy Garoppolo threw an easy pass to an uncovered Kendrick Bourne running back toward the left sideline -- exactly where Tabor would have been, had he been playing zone.

When the defense retook the field for its next drive, Tabor was benched for Lawson, and he didn't return until Slay suffered a brain injury in the third quarter.

"Gotta do a lot better," Tabor said after the game. "Nothing really sticks out. Just ready to go back to work."

Tabor's development has been slow since the Lions took him in the second round of last year's draft. There were concerns about his speed at the time, but GM Bob Quinn loved his size and intellect, and coaches have raved about that IQ since his arrival. His ability to diagnose plays and recall them later is said to be top-notch.

But that hasn't helped him mature into the cornerback they envisioned when making the pick. Tabor didn't play much as a rookie, then lost the CB2 battle to Lawson this summer. And when given his first chance to crack the starting lineup, he struggled badly.

"We'll go back at it and look at it on the film," Patricia said. "We had a couple of different packages really for both guys out there, so it was kind of a planned thing to be able to play as many guys as possible. It was a little bit warm out here. The sun was in our bench early, so I wanted to make sure we kept it moving from that standpoint, and show them a couple different matchups."

The season has not gone well for Lions second-round picks. Tabor, from 2017, got benched. A'Shawn Robinson, from 2016, was a healthy scratch in the opener. Ameer Abdullah, from 2015, was a healthy scratch for both games. Kyle Van Noy, from 2014, isn't even with the team anymore. And Slay, from 2013, has left both games after taking shots to the head, and didn't return to this one.

And with a date with Tom Brady on deck, you can bet his status will be closely monitored this week.