The 49ers placed running back Marcus Lattimore on the reserve non-football injury list today, meaning the 2013 fourth-round pick will be sidelined for at least the first six games of the regular season.

The move wasn’t surprising: Lattimore hasn’t played since he sustained a serious knee injury at South Carolina in October 2012.

The move was made as the 49ers trimmed their roster from 90 players to 75 in advance of Tuesday’s 1 p.m. deadline.

As expected, the 49ers also placed All-Pro inside linebacker NaVorro Bowman on the reserve/physically unable to perform list because of the knee injury he sustained in the NFC Championship Game. Bowman, who will also be sidelined for the first six games, is expected to miss about half the season. Tight end Garrett Celek (back) and rookie defensive tackle Kaleb Ramsey (undisclosed), a seventh-round pick, were also placed on the PUP list.

The 49ers placed three draft picks – fullback Trey Millard, cornerback Keith Reaser and guard Brandon Thomas – on the non-football injury list. The trio isn’t expected to play this season because of knee injuries they sustained in college.

The 49ers also released wide receiver David Reed and waived quarterback McLeod Bethel-Thompson, safety D.J. Campbell, tight end Kevin Greene, running back Jewel Hampton, punter Colton Schmidt, linebacker Kion Wilson and wide receiver Devon Wylie.

Finally, the 49ers released linebacker Blake Costanzo from injured reserve.

After the sixth regular-season game, the 49ers have three weeks to allow players on the NFI and PUP lists to practice. Once a player begins practicing, he can be moved to the 53-man roster at any time during the next three weeks. At the end of the three-week practice window, the 49ers can elevate him, release him or officially end his season by keeping him on the injury list.