The 10-year-old lost his long unbeaten streak first time out this season, when succumbing to Cyrname as he stepped up in distance at Ascot in November.
Henderson felt under pressure to do something different, after Altior had dominated the two-mile scene for so long, but admits with hindsight it was the wrong thing to do - and the dual Champion Chaser has since missed out on races he would normally have been expected to win.
"We said we would have a go at a longer distance, but what we did was leave all the other races like the Clarence House and Tingle Creek going begging," said Henderson.
"It was the wrong thing to do, but there was a lot of pressure put on us, saying 'Come on, do something different. If you say he is a champion, prove he can stay. Otherwise he is just a boring two-mile chaser'. So, he's a boring two-mile chaser.
"Remember when Frankel moved up two furlongs and everybody said, 'My God, can he get it?'. He was brilliant at York, and he was fantastic, but we were going five furlongs further in bottomless ground - and we were not prepared.
"I admit that, and it was my fault. He was as fit as I could get him first time out, but you don't normally expect to come into that sort of clash on your first appearance.
"I respected what people wanted. To be fair, (jockey) Nico (de Boinville) was keen on the idea (of a new trip). The biggest problem was it was the first run of the year - and he got caught out.
"I did get caught out because he wasn't ready for two-miles-five in that ground at that pace. That ground at Ascot has been unseasonably horrible all winter, to be fair."
Altior then had to miss a Kempton engagement at Christmas because of an abscess on his withers.
But Henderson added: "He looks well - and all that is fine, I hope, touch wood - but these things can come and bite you.
"He is pretty ready. He does his work as well as ever."
Altior has won the last two runnings of Saturday's Grade Two, registered as the Game Spirit - firstly as a novice in 2017.
"You can go back to the season he had his wind operation (2018), and we came back in this race," said Henderson.
"It went very soft overnight, and I had a sleepless night.
"I ummed and ahhed - and that went on until about 11am in the morning.
"Politologue made the running - he sat behind him and went past him without coming off the bridle, and that was in soft ground.
"If he is going to get to Cheltenham he has got to run in the race."
Tom George was going to send Bun Doran straight to Cheltenham after his win in Altior's absence at Kempton over Christmas - but he has had a change of heart.
George said: "He is in good form and is ready to go. He is so lightly-raced and has had so few races in his career.
"If we wait for Cheltenham and run there we might not have anywhere to go after it - so we thought we would try and get a race in between. It will also tell us what we should be aiming for - either the Queen Mother or the Grand Annual. It just keeps our options open.
"If it is the Altior of old we have got no chance. But we will have to see how he is, while Bun Dorran ran well at Kempton over Christmas.
"I would say that Newbury as a track should suit him a lot more than Kempton, because it is a bit more of a galloping track.
"He has run well there before and he has already had a good season for us, winning a Grade Two, so let's hope it continues."
The wet winter has hampered Sceau Royal, and Alan King is hoping the ground continues to dry.
"The more the ground improves the better Sceau Royal will run, and he will also be suited by a return to a left-handed track," said King, on www.alankingracing.co.uk.
"He disappointed first time out in the Tingle Creek at Sandown but ran much better in the Desert Orchid at Kempton over Christmas, and he schooled well on Monday morning."
Kalashnikov, the returning Dynamite Dollars and the veteran Simply Ned complete the field.