SANTA CLARA >> Not even the sight of Deebo Samuel, George Kittle and Trent Williams back at practice could keep the 49ers’ positive vibes going long Thursday.

Backup linebacker Curtis Robinson was carted off barely 15 minutes into practice with a non-contact knee injury, and that cart later took potential starting defensive tackle Jordan Elliott to the locker room because of a foot injury.

So goes the 49ers’ injury luck in a season they’ve started 1-2 without Christian McCaffrey, last season’s AP Offensive Player of the Year. Samuel and Kittle both missed Sunday’s 27-24 loss at the Los Angeles Rams, where defensive tackle Javon Hargrave sustained a season-ending triceps tear.

Symbolizing the team’s status, that cart was parked in front of the 49ers’ locker room after practice.

“It sucks anytime you see a teammate go down,” quarterback Brock Purdy said. “I’m just trying to practice, get ready for this game and you look over and see a cart on the field. It’s, ‘All right, who is that?’ We have to keep going and finish practice.”

The 49ers host the New England Patriots (1-2) on Sunday, and Samuel’s surprising appearance Thursday could signal his availability after what was suspected to be a two-game absence because of a calf strain. Samuel did not appear to be favoring a leg in Thursday’s action, and it’s unknown if he’d promptly reclaim his starting slot ahead of Jauan Jennings, who had three touchdown catches among 11 receptions for 175 yards Sunday.

“You have to have a perspective of being grateful when you’re healthy and take advantage of your opportunity,” Purdy added. “More than anything, the culture we have here, whoever goes down, we’ve always got their back, we’ll be there for them, and will try to get them back as fast as possible.”

Purdy, by the way, said his back has responded well this week after requiring an MRI on Monday to rule out anything serious from Sunday, when he endured “cumulative” effects from a physical game that pained him afterward.

Elliott figured to enter the starting lineup in place of Hargrave. Kevin Givens also has rotated into that spot alongside Maliek Collins, with T.Y. McGill a potential practice squad call-up Sunday.

Robinson, in his fourth year out of Stanford, immediately grasped at his right knee after racing down on punt coverage. He quickly was tended to by the team’s medical staff. Safety Talanoa Hufanga and linebacker Fred Warner came over to offer well wishes, as did coach Kyle Shanahan.

Robinson has played 59 special teams snaps, and while he’d yet to debut on defense this season, that possibility was floated in light of De’Vondre Campbell’s brief benching Sunday. Dee Winters played six snaps in place of Campbell before aggravating an ankle injury that forced him out of the game; Winters resumed practicing Thursday in limited fashion.

Kittle said he is “very excited to play football” after a one-game hiatus. He was limited for a second straight day because of what he described as a Grade I hamstring strain, the effects of which impacted him last week from the Week 2 loss at Minnesota, where he was treated in the locker room for cramps.

Rookie wide receiver Jacob Cowing did not practice because of an illness. If he is unable to play Sunday, his role as the 49ers’ primary return specialist could go to practice-squad veteran Trent Taylor, if not Ronnie Bell.

Williams was kept out of Wednesday’s practice with an illness.