The bold-jumping grey, who turned 13 on New Year's Day, is critically ill after suffering an infection in a foot.
He was last seen winning the Old Roan Chase for the third time at Aintree in October but it was reported after the race that he had a problem with a foot.
The injury became infected in the navicular bone area, which required several operations, but the wound has taken far longer than anticipated to heal.
Richards said: "The old horse is battling away for his life. I spoke to our vet this morning. It has been a hell of a job trying to close this infection down.
"The navicular bone became infected and it's into the flexor tendon as well and the infection is taking some closing down. The lady looking after him has done a fantastic job.
"We've searched worldwide and we've been in contact with everybody looking for a way to treat it and this girl seems to have everything covered.
"Regarding his racing career, though, that's over. We are just trying to save the old horse now, he's still at the vets.
"I don't know if he's over the worst yet, even.
"If we can close the infection down, it will be absolutely fantastic."
Monet's Garden has been owned throughout his career by David Yates, who was left distraught by the news. He said: "I'm very sad for the horse because we were hoping to give him one or two more races and then look forward to a happy retirement, but it's not looking like that will be the scenario, unfortunately.
"This bone infection has really got a hold but, as he has done all his life, he's fighting. Whilst he's fighting, we're fighting with him.
"He's irreplaceable, I wouldn't even think of trying to replace him.
"I actually thought his best race was in the Arkle when he was only just beaten by Voy Por Ustedes trying to give him 5lb.
"He won the Ascot Chase twice and we were hoping he might have won it for a third time. He's won the Old Roan Chase three times, the Peterborough Chase, and he's also beaten Kauto Star, which not many have done.
"He beat Poquelin in the Old Roan so even though he's 13 he was as good as ever – that's because he's been looked after very well by Nicky and his daughter. He's been a star for us all."
One of a number of interesting runners at Southwell today is Knight Pass, second favourite for the Champion Bumper in March.
The Warren Greatrex-trained five-year-old created a big impression when winning at Sandown on his debut, and is as low as 14-1 for the Festival event.
Owned by Malcolm Denmark, who tasted Bumper glory with Monsignor back in 1999, Knight Pass will face 13 rivals in the southwell-racecourse.co.uk Standard Open National Hunt Flat Race.
"It is always hard to win with a penalty, but it's basically just to give him more experience," said Greatrex.
"I think he's come on since Sandown and I haven't hidden that I think the world of him and with a bit of luck he'll do the job and we can make a plan.
"If he wins he'll probably have one more run before Cheltenham. There's a good race at Newbury next month which could be a possible.
"He surprised me at Sandown because they went no gallop. If they'd gone a better gallop I think he'd have won by further.
"I don't want to tempt fate but I think he's pretty smart."
Another to keep an eye on at the Nottinghamshire track is Bellvano, who looks likely to mount a major challenge in the starsportsbet.co.uk Novices' Chase.
Nicky Henderson's charge won two out of three novice hurdles, finding only subsequent Supreme Novices' Hurdle hero Menorah too good in the other, and though he struggled in the County Hurdle at the Cheltenham Festival, he looks a potential class act.
However, the best bet of the day tomorrow could be Timesawastin, who lines up in the Southwell 'National Hunt' Maiden Hurdle.
The five-year-old found only subsequent Grade 1 winner Hidden Universe too good on his sole bumper start for Edward Hales in Ireland, and he shaped well when third on his first run for Evan Williams.
The winner that day has run a fair enough race in Grade 2 company since and the runner-up has scooped two subsequent successes, there is a lot to like about the form of that race and he can put that experience to good use.
Big Buck's will not be seen on the track again until he bids for a third consecutive Ladbrokes World Hurdle at the Cheltenham Festival.
Trainer Paul Nicholls had considered slotting in another start at Prestbury Park after he took his winning sequence over timber to 10 following another laughably easy success in the re-arranged Long Walk at Newbury.
Nicholls reported: "I have had a discussion with (owner) Andy Stewart this morning and we have decided that Big Buck's will sidestep the Cleeve Hurdle and go straight to the Cheltenham Festival.
"Taking in the Cleeve Hurdle was our original plan. But he will be given a break instead and his campaign will now mirror last season's."
Merigo's fall in the Becher Chase at Aintree has not caused Andrew Parker to have a rethink about aiming for the John Smith's Grand National.
Parker's more pressing concerns have been the bitter weather at his Lockerbie stables and whether last year's Scottish National winner is given a high enough weight to sneak into the famous race.
"He'll be entered for the Grand National and for the Scottish National again," said Parker.
"I schooled him myself a couple of days after his fall and he was absolutely brilliant again. If he runs again before the weights come out for the National it will probably be over hurdles, which won't move his chasing mark up or down."
Ludlow's meeting tomorrow has been abandoned due to a frozen track.