While the best and most important players on a roster are usually obvious, a college football team's upside can be heavily contingent on one person or position.

One familiar term to describe that player is an X-factor, which has a flexible definition worth clarifying. For us, it's a player (or position) who isn't an established superstar, yet needs to excel in an instrumental role for a team to compete for a national title.

In most cases, the focus landed on a key player returning from an injury or someone who hasn't previously held a featured spot. But a couple of very familiar names are considered, too.

Organized alphabetically by school, the list is subjective and considers a mix of past performance and 2021 role.

Alabama's Running Backs

Alabama has several X-factor possibilities considering the offense must replace five first-team AP All-Americans. Given that quarterback is the most important position, the transition from Mac Jones to Bryce Young has drawn the largest share of the headlines.

But the X-factor is the rest of the backfield.

In theory, the Crimson Tide should be able to quickly move on from Najee Harris. They have former 4-star Brian Robinson Jr., top-50 prospect Jase McClellan, 5-star Trey Sanders, 4-star Roydell Williams and 5-star Camar Wheaton as the next running backs in line.

Replacing 1,466 rushing yards, 43 catches for 425 yards and 30 total touchdowns is much easier said than done, though.

Alabama has other questions, but the answers at quarterback (Young), their top target (John Metchie III) and on the offensive line are more straightforward. Finding the right combination of running backs can take the offense from good to great.

Justyn Ross, WR, Clemson

Based on past performance, Justyn Ross is closer to a superstar than an X-factor. During the 2018 and 2019 seasons, he collected 112 receptions for 1,865 yards and 17 touchdowns. He is, without question, one of the nation's most talented receivers.

The concern for 2021 is his health.

After undergoing surgery for a congenital fusion in his spine and missing the 2020 season, Ross is cleared to return. We hope Clemson's star wideout will regain his previous form, but nobody can know the answer with any certainty.

If yes, Ross fills the "proven top receiver" box otherwise unchecked on the roster. If no, however, would quarterback D.J. Uiagalelei be able to carry an offense of decent targets? Would another receiver, such as E.J. Williams or Frank Ladson Jr., develop into that key option?

Clemson is the ACC favorite either way, but the best version of this offense includes Ross playing like an All-American.

Arik Gilbert, WR/TE, Georgia

Depending on who you ask, you'd probably find several perceived X-factors in Georgia's receiving room.

Marcus Rosemy-Jacksaint and Dominick Blaylock are both returning from injuries. Blaylock, in particular, is a solid choice considering he caught 18 passes for 310 yards and five touchdowns as a freshman in 2019. But our focus is on Arik Gilbert.

While at LSU last season, he made 35 receptions for 368 yards and two scores. Gilbert transferred to UGA, where he's expected to help fill the void of top receiver George Pickens (torn right ACL).

The challenge? Gilbert is sort of a tight end.

Listed at 6'5" and 248 pounds, he's officially a wide receiver on the roster. Georgia is likely to move Gilbert around, though, and his elite versatility translating to elite production is vital for the Dawgs to truly contend for a national title.