Midfielder Bacuna appeared to make contact with the match official on the touchline, as he contested a decision, and referee Andrew Madley gave the 25-year-old his marching orders.
Bruce said: “The linesman got the decision horribly wrong as it was a throw-in for us. It was a huge frustration but he overstepped the mark and he knows that.
“It was kind of a chest bump and it was disappointing as he had given a brilliant performance.
“But with Leo it is all about himself. He always tests the manager as he makes a habit of being late, but against Derby he reminded what a good player he is.”
Bacuna could face an extended ban, depending on the interpretation of the incident.
Aside from that, Bruce was beaming after watching Villa finally win a game in 2017.
It was a hard-fought success against a Derby side who dominated for long spells.
The turning point was a first-half incident in which Nathan Baker appeared to foul Derby’s former Villa striker Darren Bent.
Although both Villa boss Bruce and Derby counterpart Steve McClaren thought it was a blatant penalty, referee Madley gave nothing.
Bruce was just happy with the result which lifts the gloom around Villa Park.
Bruce said: “It was a relief for everyone. It was always going to be difficult to get us up and running. Fortunately we got a result and this is the only thing that matters in times like this.
“We defended well and worked hard. We had two or three opportunities to make it 2-0 but with the mob I’ve got it was unlikely they would make it easy.”
The Villa manager paid rich credit to captain James Chester, who headed the only goal in the 25th minute.
It was the Wales international’s first goal since his move from West Brom.
“Chester was terrific,” said Bruce. “It was more like James Chester.
“He put in a real captain’s performance and scored a goal which is a rarity in itself. It is only our second goal from a corner which is ludicrous.”
Bruce added: “This result will do my goalkeeper Sam Johnstone the world of good. I think he is an exceptionally good young goalkeeper. He has come in at a difficult time and this was his first clean sheet in eight starts.”
Derby suffered a big blow when they lost skipper Richard Keogh to injury early in the second half.
Beaten boss McClaren said: “This was another bizarre game. I don’t know how we lost. We were dominant from start to finish and it was an excellent second half.
“We did everything but score but all matches are hinged on decisions. It was a stonewall penalty and it has won them the game.
“Had we had the opportunity to score it would have been a different result.
“The last few games have not gone for us but if we continue playing like we did against Villa we will win games. We are capable of going on a run.”