Tight end Garrett Celek's concussion is among many injury concerns for the 49ers. Mark J. Rebilas USA TODAY Sports

The struggling San Francisco 49ers could be severely undermanned heading into Sunday's matchup against the visiting Atlanta Falcons.

Starting running back Carlos Hyde (foot) and leading wide receiver Anquan Boldin each sat out practice Wednesday with injuries that caused them to miss Sunday's 27-6 loss at St. Louis. Coach Jim Tomsula listed both players as day-to-day.

More from FoxSports

With backup running backs Reggie Bush and Mike Davis each undergoing surgery on Tuesday, San Francisco could roll out a backfield of three players who were not on the team's active roster prior to last Saturday.

Kendall Gaskins was promoted from the practice squad on the eve of Sunday's game and wound up rushing for six yards on five carries. The 49ers signed free-agent running backs Pierre Thomas and Shaun Draughn this week.

Tight end also is looming as an issue after San Francisco shipped Vernon Davis to the Denver Broncos at Tuesday's trade deadline. Garrett Celek, who started while Davis was sidelined with a knee injury, suffered a concussion in Sunday's game.

That leaves third-year tight end Vance McDonald, who has seven catches in seven games, and converted quarterback Blake Bell as the only options at the position. Meanwhile, safely L.J. McCray underwent knee surgery on Tuesday while cornerback Kenneth Acker is also dealing with a concussion.