SANTA CLARA – Brock Purdy injured his throwing elbow again, the 49ers’ most damning fallout from a 40-34 loss to the Detroit Lions in Monday night’s home finale.

“It doesn’t feel the best,” Purdy said. “Basically, my arm was on fire. I tried throwing a couple on the sideline and I couldn’t. … We did some tests with the UCL and I was good in that regard.”

Purdy expects an MRI on Tuesday to reveal the extent of his injury. This comes 23 months after his ulnar collateral ligament was torn in the 49ers’ NFC Championship Game loss at Philadelphia, and it’s one week until Purdy becomes eligible for a contract extension expected to fetch him $50 million or more annually.

“We think it’s some nerve stuff,” coach Kyle Shanahan said. “I wouldn’t think (long-term). … Just talking to doctors, he got hit in the right spot, and when you get hit there, your nerves go, you can’t feel much. Hopefully it’s alright.”

Purdy said he “100 percent” wants to play in Sunday’s regular-season finale at the Arizona Cardinals (7-9). The 49ers (6-10) clinched last place in the NFC West courtesy of this sixth loss in seven games.

Regardless of this game’s outcome, the Lions (14-2) knew their postseason path would be tied to the NFL’s regular-season finale Sunday night, when they host the Minnesota Vikings (14-2) in a winner-take-all matchup for the NFC North title and No. 1 playoff seed.

Purdy played well initially behind a patchwork offensive line featuring newcomers at left tackle and left guard. He had a perfect, 158.3 passer rating at halftime (14-of-16, 200 yards, two touchdowns) but a pair of second-half interceptions torpedoed a mistake-prone team that’s lost so often with late-game miscues.

“It seemed like he had a really good night except for those two picks, and they were two tough ones,” Shanahan said of Purdy, who finished 27-of-35 for 377 yards with three touchdown passes and two interceptions.

This 49ers’ home finale opened with so much of what The Faithful restlessly sought this season: Purdy in MVP-caliber form; Deebo Samuel scoring a touchdown; George Kittle bulldozing defenders; first-round pick Ricky Pearsall making grabs on a breakout night (eight catches, 141 yards); and, Nick Bosa harassing the opposing quarterback.

Alas, the 49ers could not keep pace with the Lions, nor rally past them like last matchup, when they overcame a 24-7 halftime deficit and won the NFC crown 34-31.

“The difference in the game, in the type of game it was — and they’ve been in a number of them this year — we had to win it in a shootout. The difference was our two turnovers,” Shanahan said.

Eliminated from playoff contention eight days earlier, the 49ers’ familiar mistakes arose. Purdy got intercepted twice, Jake Moody missed three kicks, the 49ers defense produced no interceptions, two rushing touchdowns were allowed, and penalties further hurt the Niners’ chances.

When Purdy threw a fourth-quarter interception (for the sixth time this season), the 49ers were at the Lions’ 36-yard line and trailed 34-28 with 7 1/2 minutes remaining. The comeback essentially died there, all due respect to a 7-yard, last-minute touchdown run by Josh Dobbs, Purdy’s replacement and Sunday’s potential starter if Purdy is out and the 49ers’ offensive line remains in shambles.

Dobbs’ score set up more misery for Moody, who missed the ensuing point-after attempt for his third miss of the night. A 51-yard field-goal attempt was no good as the 49ers took a 21-13 lead into halftime, and a 58-yard try would hook wide on the drives in between Purdy’s interceptions.

How concerned are Purdy’s teammates with his health? “I’ll be concerned tomorrow, if I need to be,” said Kittle, who had 112 yards (eight catches) to raise his team-leading season totals to 1,079 yards on 76 catches with eight touchdowns.

The Lions’ Jared Goff, a Marin County native and Cal product, passed 303 yards and had three touchdown tosses, including a go-ahead, 4-yard strike to Aman-Ra St. Brown, 19 seconds before the fourth quarter, for a 31-28 lead.

“It’s too many ‘My bads,’ everybody having their one play of (not) being on it,” linebacker Fred Warnder said. “I’ll always look at myself in how I can do better bringing everybody along.”

The 49ers’ attempted counterpunch: a 58-yard field goal try that hooked wide by Moody, who is 10-of-18 since returning from a high-ankle sprain.

Purdy’s third touchdown pass of the night also was a go-ahead throw, putting the 49ers up 28-21 midway through the third quarter. That 5-yard score came on an old playbook go-to — a short flip to an in-motion Deebo Samuel. The Lions had just pulled even at 21 on a Goff touchdown pass to Sam LaPorta and an ensuing two-point conversion pass.

Purdy threw touchdown passes on the 49ers’ first two drives, then scored himself on their third series, scrambling nine yards up the middle for a 21-13 lead 6:13 before halftime. Purdy was 9-of-9 passing for 131 yards and a perfect 158.3 passer rating to that point, having just completed a 40-yard bomb to Pearsall at the Detroit 9-yard line to beat a third-down blitz.

Bosa played his fiercest game of the season: two sacks, four quarterback hits, and eight tackles (four for loss). Bosa required medical clearance from a potential concussion after his first sack, and he thrived against Lions star tackle Penei Sewell.

But the 49ers’ defense couldn’t muster more to prevent Detroit from scoring 40 points for a sixth game this season.

Purdy’s night hit its first setback when Kerby Joseph intercepted him with 4:11 left in the third quarter, on an overthrow toward Pearsall over the middle, with the 49ers ahead 28-24. (Joseph also intercepted Purdy on the previous snap that was nullified by a pass-interference penalty).

The Lions, like the 49ers, scored touchdowns on their first two drives of the night (both by Jameson Williams). They went for it on fourth down to try matching the 49ers’ touchdown hat trick. However, rookie Renardo Green broke up Goff’s throw into the end zone, preserving the 49ers’ 21-13 lead 1:13 before halftime. Purdy shrewdly used that time to usher the 49ers into scoring position. Moody couldn’t score, however. He missed a 51-yard field goal attempt two seconds before halftime.

Held under 20 points in losing five of their previous six games, the 49ers owned a 21-13 halftime lead.

On the 49ers’ second touchdown drive, Kyle Juszczyk scored on a 9-yard touchdown pass from Purdy, giving Juszczyk at least one touchdown catch in each of his eight seasons as their fullback. On the preceding play, George Kittle had a 20-yard catch-and-run that pushed him over the 1,000-yard mark for the fourth time in his eight seasons.

That 14-6 lead lasted less than five minutes, as Williams scored the Lions’ second touchdown on a 42-yard hook-and-lateral from St. Brown. Two snaps earlier, Bosa recorded his eighth sack of the season, showing relentless pursuit after an inside move beat Sewell, who got called for holding Bosa the previous play; Bosa was cleared after a concussion evaluation following the sack.

Shanahan’s opening script worked to 7-point perfection: an 11-play, 61-yard drive that saw Purdy complete 3-of-3 passes, including a 3-yard scoring strike to Pearsall that was awarded after via officials’ replay. Purdy converted on a fourth-and-1 sneak two snaps earlier from the 5-yard line to keep alive that series.

Kittle’s final words of the night summed up the 49ers’ demise as NFC champions: “We have to go back to doing all the little things right before we do the cool, big things well.”