SANTA CLARA, Calif. (AP) San Francisco 49ers rookie center Marcus Martin suffered a left knee injury that isn't considered season-ending, yet still a blow to the depth of a struggling offensive line.
The third-round draft pick was carted off the field with 11:18 remaining during a 21-7 preseason victory Sunday against San Diego.
''He had an MRI this morning and, can't go into all of the details because I don't know them all, but it doesn't appear that it's an ACL,'' coach Jim Harbaugh said Monday. ''I don't want to put a timeline on it right now.''
Asked whether Martin could return sometime in 2014, Harbaugh said: ''It appears that way, yes. We'll just hold off on any definitive comments.''
Martin already dealt with an ankle injury that sidelined him early in training camp. The 70th overall pick out of Southern California, Martin played 38 games - with 33 starts - over three seasons with the Trojans.
The 49ers are looking to make some quick adjustments on their struggling offensive line, which showed its vulnerabilities in pass protection as quarterback Colin Kaepernick took several hard hits and was constantly pressured Sunday.
Harbaugh again declined to speak about holdout right guard Alex Boone, who would be a big help but is still away from the team while seeking a new contract with two years remaining on his previous deal. He might have gained some leverage with Sunday's game.
The 49ers have a quick turnaround for Thursday night's preseason finale at Houston to get things right on the offensive line.
''Just continuing to get reps, get reps at the looks that we're going to see,'' Harbaugh said. ''In a preseason game where it's going to be a short week that'll be difficult to do, but we'll attack that.''
Fourth-year pro Daniel Kilgore is gearing up for his first season as starting center. Harbaugh praised him Monday for not missing a call.
For years, Kilgore has been among the first players on the field for game day warmups going through his extensive routine to prepare for this opportunity.
''He's a very athletic center, good combination of strength and quickness,'' Harbaugh said. ''He's got a quick mind. He's done a very good job with making the calls and understanding the system. And he's on a streak of not missing a call now for . I can't remember the last time he missed a call. It's been good. We're very pleased. Knock on wood. He's continuing to get those reps and take care of his business.''
Kilgore figures more emphasis on technique this week and some other slight adjustments from everybody will have the O-line on the same page and doing a better job giving Kaepernick enough time to do this thing - without going to the ground as often.
There has already been learning from the game film, Kilgore said. He appreciates the confidence from Harbaugh, too.
''I take it as a compliment. If you make a mistake out there or if I miss calls, you have a tendency to forget them and move on,'' Kilgore said. ''This is four years I've been with this offense. I'm ready to go.''
Harbaugh said wide receiver Stevie Johnson is likely to make the final 53-man roster announced Saturday.
''Stevie's done an outstanding job,'' Harbaugh said. ''I feel very good about him being on the 53-man roster, yes.''
The 49ers on Monday removed the temporary sod from Sunday's game to make modifications to the base - soil and sand - and create a better root system that should hold for much of the remainder of the season and only require replacing on an on-need basis. The new sod will go down by the weekend to have a full week to set in before a soccer match between Chile and Mexico on Sept. 6.
''They're starting over,'' punter Andy Lee said. ''I'm sure it will be great. I'm not worried.''
NOTES: In Monday's roster moves, San Francisco waived QB McLeod Bethel-Thompson, S D.J. Campbell, TE Kevin Greene, RB Jewel Hampton, P Colton Schmidt, LB Kion Wilson and WRs Devon Wylie and David Reed. The 49ers placed RB Marcus Lattimore, FB Trey Millard, CB Keith Reaser and G Brandon Thomas on the reserve/non-football Injury list. LB NaVorro Bowman, TE Garrett Celek and DT Kaleb Ramsey were placed on the reserve/physically unable to perform list and LB Blake Costanzo was released from the injured reserve list.
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