It makes sense for Falcons coach Mike Smith to emphasize his “next man up” theme as his team deals with a sudden string of injuries to key players.

It’s just that, realistically, the next men up won’t be as good as the ones who went down. The Falcons, like all teams, put what they consider their best players on the field for the majority of the plays while those they’ve determined are not as good serve as backups.

That doesn’t necessarily mean the reserves can’t play well enough to help the Falcons win. It does mean the Falcons (1-1) will lean on several backup-caliber players with two starters out for the season, another out for at least eight weeks, and yet another out for Sunday’s game at Miami (2-0).

Linebacker Sean Weatherspoon is the latest key player to be sidelined by a major injury. He will miss at least eight weeks with a foot injury suffered Sunday against the Rams after the Falcons placed him on short-term injured reserve Tuesday.

Coach Mike Smith said running back Steven Jackson won’t play Sunday at Miami after he pulled a thigh muscle against the Rams. NFL.com reported that Jackson could miss up to four weeks, but Smith wouldn’t provide a time frame except to say “it’s short-term, we think.”

Do-everything defender Kroy Biermann and fullback Bradie Ewing already were ruled out for the season. Another starter, cornerback Asante Samuel, could miss the Dolphins game after he lasted only seven plays against the Rams before aggravating a thigh injury that sidelined him the previous three weeks.

With so many important veterans injured, the Falcons have no choice but to rely on several inexperienced players in key roles.

“Being a backup player is probably the toughest job in the NFL because you don’t get the reps during the week, and then all of a sudden you are one play from going in,” Smith said Tuesday. “And when you go in everybody is expecting you to play at the same level as the guy in front of you. I think our guys — I know our guys — are prepared.”

The Falcons appear in good shape to replace Jackson in the short term and Ewing for the long haul with veterans Jacquizz Rodgers and Jason Snelling. Things don’t look so tidy on defense.

Biermann filled a hybrid pass rush/coverage/run-support role for the Falcons. The most likely candidate to replace him is Jonathan Massaquoi, who logged a handful of defensive snaps as a rookie in 2012.

Falcons defensive coordinator Mike Nolan said Massaquoi is “one of the guys” who will fill in for Biermann, but that Nolan will fit the scheme to those players who now are in more prominent roles.

Weatherspoon led the linebackers in snaps before his injury. He’s also the defensive captain, responsible for making sure the players on the front line up correctly.

Two rookie linebackers who were not drafted, Joplu Bartu and Paul Worrilow, are next in line. Veteran linebacker Stephen Nicholas fell behind after suffering a leg injury during training camp and played only four defensive snaps against the Rams, as Bartu took over for Weatherspoon.

Moving Weatherspoon, Biermann and Ewing to injured reserve opened three active roster spots. The Falcons filled two of those slots with veteran free-agent linebackers Jamar Chaney and Omar Gaither.

Chaney, 26, started 23 of 44 games he played for the Eagles the past three seasons. Gaither, 29, started 44 of 84 games he played from 2006-12 for Philadelphia, Carolina and Oakland.

“We are very comfortable right now with the young (linebackers) that played in the preseason that understand our system, and we’ve got to get these (new) guys up to speed as quickly as possible,” Smith said.

The Falcons also signed Patrick DiMarco to the active roster from their practice squad and signed linebacker Chase Thomas to take DiMarco’s place on the practice squad.

The relatively positive news for the Falcons is that they can look forward to the return of Jackson and Weatherspoon this season.

Smith said there was hope that Jackson might recover enough from his injury to make it possible he could play this week.

“I don’t know if (short term) is two or three weeks,” Smith said.

Smith said Weatherspoon’s injury “is not anything that’s going to keep him out more than eight weeks.” He said Weatherspoon should be ready to practice in six weeks, when he’s first eligible by NFL rule.