Thompson's addition to Hird's panel was announced on Wednesday after weeks of speculation following his decision to leave Geelong, citing exhaustion with senior coaching.
Hird said his former teammate had not done anything wrong in accepting a role at the club where he played 202 games and was a premiership captain.
"Mark has given two premierships and 11 great years of service to Geelong and I can understand why Geelong are disappointed because I'd be disappointed if Essendon lost someone of the quality of Mark Thompson," Hird told SEN on Thursday morning.
"They've got every right to be disappointed he's left but I don't think they've got the right to feel he hasn't done the right thing by them, because for 11 years he gave his heart and soul to the club and did a great job in winning two premierships for them."
Hird said Thompson's unannounced arrival at Wednesday's training session at Penleigh and Essendon Grammar in Keilor Park was one of the most bizarre situations he had been a part of.
"We were running a drill, the players were running back and forward and there's Bomber standing in the middle of the oval," he said.
"I thought, 'What's going on?', and he said, 'Let's go, let's do it'."
After officially accepting the position, Thompson briefly addressed the players before returning to his holidays.
Hird conceded he had spoken to the two-time premiership coach more recently than he claimed on Monday, when he said it had been a few weeks since they talked.
"We'd spoken last week that he was keen to do it but the absolute definite 'I'm going to do it', wasn't until yesterday," he said.
"Mark's his own man; he makes his own decisions in his own time.
"People have speculated but until Mark in his own mind decided that he was going to do it, I think we were all a bit in the air."
Hird said it was an emotional moment for the club and those involved, particularly for the trainers and support staff who have worked at Windy Hill since Thompson was a player.
He also said Thompson's role was yet to be defined but would involve supporting the coaching staff, offering advice on the game plan and teaching the players how to play.
"It's very exciting for me personally but on a much bigger scale it's just great for the Essendon football club," he said.
"I think the players are very excited about having someone of the quality of Mark in and around the club and teaching them how to play football.
"If you're an Essendon supporter, you should be very excited by what the club has put together and is trying to do to get us back into the finals."
