We’ve all heard it: It takes several seasons to properly evaluate an NFL draft class.

And it might take even longer to get a handle on the 49ers’ class of 2015.

Despite a roster dotted with more holes than in past seasons, general manager Trent Baalke selected several raw projects with unique backgrounds. The trend began with first-round pick Arik Armstead, a defensive tackle who wasn’t a full-time football player until his final season at Oregon because he also played basketball.

Even in May, it appeared evident that none of the 49ers’ 10 picks would open the season as a starter.

Given that, this shouldn’t come as a surprise: Safety Jaquiski Tartt is the only rookie to play more than 21 percent of snaps this season, and he is logging significant playing time only because starter Antoine Bethea is on season-ending injured reserve.

Here’s a rundown of the draft picks after nine games:

DT Arik Armstead, first round

Defensive snaps played: 21 percent (130 of 620)

Armstead has been used as a situational player on passing downs and consistently has produced pressure with a strong bull-rush move. On 114 passing snaps, Armstead has collected a sack, three quarterback hits and 17 hurries, according to Pro Football Focus. Defensive coordinator Eric Mangini recently said Armstead has made notable strides after his early-season performance included some habits Mangini wasn’t “excited about.” There are reasons to be excited about Armstead, whose size (6-foot-7, 292 pounds) and agility should be on display more in the final seven regular-season games.

S Jaquiski Tartt, second round

Defensive snaps played: 34 percent (210 of 620)

One year removed from playing teams such as Concordia and Wofford, Tartt has made a rather seamless jump from small-college football to the NFL. The Samford alum replaced starter Bethea in the past two games and has flashed excellent range while providing several huge hits, including a wallop on Atlanta wide receiver Julio Jones. Tartt’s aggressiveness cost him in his first start against St. Louis. He sprinted up from the secondary to make plays and was out of position on a 71-yard touchdown run and a 66-yard scoring pass. Still, Mangini said Tartt’s fearlessness is an excellent trait and can be harnessed. In Mangini’s view, aggressiveness can’t be coached: (If) “they don’t bite when they’re puppies,” he said, “they don’t bite when they’re older.”

LB Eli Harold, third round

Defensive snaps played: 20 percent (122 of 620)

Give this rookie some more time in the weight room and cafeteria. The release of Aldon Smith in training camp provided Harold with an opportunity to receive meaningful snaps, but he’s not quite ready to provide meaningful production. One reason: Harold, 6-3 and 247 pounds, is shrimp-sized for a 3-4 outside linebacker. Starters Ahmad Brooks (259) and Aaron Lynch (270) have more than 10 pounds on him, and Harold’s lack of strength makes him a one-dimensional speed-rusher. His lack of go-to moves explains his scant production: no sacks and five tackles.

TE Blake Bell, fourth round

Offensive snaps played: 13 percent (66 of 569)

The 49ers have traded three tight ends (Vernon Davis, Derek Carrier and Asante Cleveland) in the past three months, which has cleared a path for Bell to make an impact. Eventually. His scant playing time and production (three receptions, 17 yards) can be explained partly by his unique background. Bell, who was a quarterback at Oklahoma, didn’t switch to tight end until his final college season, when he had 16 receptions. He’s the No. 3 tight end behind Garrett Celek and Vance McDonald.

RB Mike Davis, fourth round

Offensive snaps played: 12 percent (66 of 569)

It has been a rough NFL introduction for Davis, who had surgery for a broken hand Nov. 3 and landed on injured reserve with the designation to return. Before his injury, Davis had produced a painful stat line: 25 carries, 24 yards. If he returns this season, he’ll have a chance to avoid a dubious distinction. Since 1960, only one running back has averaged less than a yard a carry with at least 25 carries in a season (Donnell Bennett, Chiefs, 2000). Davis (5-9, 217 pounds) hasn’t been assisted by a below-average offensive line, but he also hasn’t shown memorable traits. He doesn’t have great speed or make-defenders-miss ability.

WR DeAndre Smelter, fourth round

Smelter, who is on the non-football injury list, began practicing this week, but he’s unlikely to join the 53-man roster after he sustained a torn ACL last year. Baalke’s penchant for drafting players recovering from serious knee injuries hasn’t been successful. The list: defensive tackle Tank Carradine, running back Marcus Lattimore, guard Brandon Thomas, cornerback Keith Reaser and fullback Trey Millard. Even when he’s ready, Smelter will require patience. He played only baseball in his first two seasons at Georgia Tech.

P Bradley Pinion, fifth round

By drafting Pinion, 21, and subsequently trading All-Pro Andy Lee, 33, the 49ers got younger and cheaper at the position. They’ve gotten what they paid for. Pinion ranks 24th in the NFL gross average (44.4) and 18th in net average (39.9). Lee ranks sixth in both categories. Pinion, the youngest player on the team, was the only punter drafted. This month, special-teams coach Thomas McGaughey said Pinion’s underwhelming rookie season can be attributed partly to problems performing under pressure: “A lot of kickers can kick the ball when nobody is out there,” McGaughey said. “… But when you put him in front of 100,000 people, it’s a little bit different, and the pressure is there.”

G Ian Silberman, sixth round

Offensive snaps played: 0

Silberman started the third preseason game, but has been inactive for eight of nine regular-season games.

OT Trent Brown, seventh round

Offensive snaps played: 2 percent (9 of 569)

He’s 6-8 and 355 pounds, and left tackle Joe Staley insists he hasn’t had a more titanic teammate. Brown is a project who started 11 of 23 games at Florida, but he has surprisingly nimble feet, and he made enough progress in practice to earn a nine-snap cameo at right tackle Nov. 8 in his NFL offensive debut. Starter Erik Pears isn’t a long-term answer, and Brown could compete for the 2016 starting spot if Anthony Davis doesn’t return to the team.

TE Busta Anderson, seventh round

Anderson is on injured reserve with an ankle injury he sustained in the preseason.

Eric Branch is a San Francisco Chronicle staff writer. E-mail: ebranch@sfchronicle.com Twitter: @Eric_Branch