The Scot enjoyed 11 successful years as Everton boss, but he angered fans of the Merseyside club with his botched attempt to sign Leighton Baines and Marouane Fellaini together on the cheap last summer.
The Everton faithful barracked Moyes when the they won 1-0 at Old Trafford in December, and it was the same story on Sunday as they chanted "sacked in the morning" at their former manager, who was also booed when he emerged from the tunnel.
One fan even dressed up as the Grim Reaper to poke fun at the United boss from just behind the dugout.
Howard, signed by Moyes in 2007, has not forgotten what the former Preston manager achieved at Goodison Park and he feels the verbal abuse that came from the stands this weekend was wrong.
"We have brilliant fans, but that was unfair on him," the Everton goalkeeper told Press Association Sport.
"He was the second coming, the messiah 12 months ago, and I don't think as Evertonians we should ever forget that."
When Moyes replaced Walter Smith as Everton manager 12 years ago the Toffees were one point above the relegation zone.
By the time he left, Moyes had given the club its first taste of Champions League football and he also made it to the FA Cup final in 2009.
Yet the three-time manager of the year is now in danger of seeing his reputation tarnished thanks to a dreadful maiden campaign at Old Trafford.
United, seventh in the league, have not qualified for the Champions League for the first time in almost two decades, but Howard thinks it would be wrong to brand the United boss a failure.
"I love David Moyes. I think he is a brilliant manager," said the American, who spent four years at United.
"He was (a brilliant manager) here for us and I will be forever grateful to him.
"Football is a funny game. It has highs and lows. You take the good with the bad and you keep rolling."
While some Everton fans have not forgotten what Moyes did for the club, it would be hard to find a supporter who does not think that his replacement Roberto Martinez has taken the club on to another level this year.
Everton have played some of the most inventive, attacking football this season under the Spaniard, who has used the loan market brilliantly.
Howard thinks Everton have made good progress under their new manager.
"We went into the United game expecting to win. That says a lot about how far we have come this season," said the 35-year-old, who signed a contract extension at Goodison Park last week.
"We were probably expected to win and that was a positive sign for us."
One of those games is against title contenders Manchester City, but Howard believes Everton can still end the season in the top four.
"I think City will know what they are in for," Howard said.
"Fourth place is achievable. We want to go into that last game still feeling like we are 90 minutes away from Champions League football."
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