Tiger Roar has been scratched from Saturday's main race after his troublesome knee acted up again.

The likely favourite for the $100,000 Class 1 race (1,600m) wrapped up a routine gallop without any noticeable issues on Tuesday, but became mildly lame a few hours later.

Trainer Michael Clements must have felt a sense of deja vu. Just over a year ago, the son of Wandjina had issues with the same near-fore knee after a race.

Knee chips were then diagnosed and duly flushed out. Clements and Thai champion owner Falcon Racing Stable were rewarded for their patience when Tiger Roar returned with a vengeance 10 months later.

At his new campaign, the 2021 Champion Three-Year-Old has racked up two wins from four starts.

He fluffed his lines in the Group 1 Singapore Derby (1,800m), but bounced back at his last start on Aug 28 with a rousing win in a Class 2 race over 1,400m.

The Group 1 Queen Elizabeth II Cup (1,800m) on Oct 19 and Group 1 Singapore Gold Cup (2,000m) shimmered in the distance, but they were not meant to be.

"It's quite serious. He reinjured his left knee after his Tuesday gallop," said Clements.

"He actually came back okay, he was quite comfortable. But I guess the adrenaline was still there, because later in the morning after he cooled down, he started to favour a leg.

"We had him X-rayed and our worst fears were confirmed, except that he had fractured a bone to the knee this time.

"Last time, he had bone fragments to the same knee. They had to be taken out and he successfully returned to racing.

"He will again have to undergo surgery, except that they will have to put a screw in the bone this time.

"It's a small bone in the knee, it's not a high weight-bearing bone, similar to a wrist joint. So, he's actually quite comfortable in himself."

Clements is aware that such a recurring Achilles' heel can hamper a horse's racing career. But he remained upbeat it had not spelled the end of a record of seven wins and two placings from 15 starts.

"He's actually really sound in his prep and training. He's got a conformation issue where the weight pushes onto that knee," said Clements.

"With more and more gallops and racing, wear and tear takes its toll, and unfortunately, it's given way again.

"I hope he can come back again next year. We just have to look after him."

Clements is rueing Tiger Roar's withdrawal, but he is still left with a sizeable team of four runners in the Class 1 contest - Top Knight, Big Hearted, Prosperous Return and Tangible.

Quite justifiably, the Zimbabwe-born 2020 Singapore champion trainer had made Tiger Roar his leading hope, be it for Saturday or the two Cup features.

But the four-pronged back-up is by no means in there for the numbers, although Clements already has a fair idea who will be the new flagbearer.

"Prosperous Return has done nothing wrong this prep. He ran really good last time," he said.

"He would be the stable elect."

The Tivic Stable-owned Fighting Sun five-year-old may not have annexed a feature race to his resume yet, but the six-time winner (1,200m to 1,600m) is a progressive sort who will not be out of his depth in the QEII Cup and Singapore Gold Cup.

In the last race Tiger Roar won, he was making inroads under Jake Bayliss down the middle of the track, only to find one better in his stablemate.

Bayliss takes the reins again on Saturday.