Eden Hazard should not have "taken the law into his own hands" when he tried to retrieve the ball from a ball boy, according to Professional Footballers' Association chairman Gordon Taylor.
The Chelsea midfielder was sent off for kicking Swansea ball boy Charlie Morgan during the Capital One Cup semi-final second leg.
"You can't take the law into your own hands," the PFA chief told BBC Sport.
"He lost his head and had to receive a punishment."
Taylor added: "You can understand the frustration of wanting to get the ball back but his actions were unacceptable and the referee had no alternative."
The Football Association is reviewing footage of the incident.
Hazard was dismissed for violent conduct, which normally carries a three-match ban, although the penalty can be increased "in "exceptional circumstances".
However, both former Chelsea striker Pat Nevin and current Stoke forward Michael Owen said the ball boy was also at fault.
Owen was one of several players to convey his opinion on Twitter. The former England forward said the ball boy's reaction to the incident was "scandalous".
"I maintain my opinion from last night. How people can claim Hazard assaulted a kid is embarrassing," said the 33-year-old, who has scored 40 goals for England.
"He shouldn't have kicked the ball out of his grasp, but he hardly booted the lad as some people were suggesting. The lad's antics were scandalous and no wonder he isn't taking any further action."
Nevin said he would have reacted similarly to Hazard.
"I'm very, very disappointed with how the ball boy acted. I say acted, he must have watched footballers the way he rolled around pretending to be more injured," the Scot told BBC Radio 5 live.
"He's only got one job and that's to give the ball back. What does he do? He keeps the ball.
"He's 17 not 12 or 13. He should know what his actions are like in this situation.
"We're actually going to have to get rid of ball boys - that's a real sadness. I would have kicked the ball out from under the ball boy - 100%. Sorry, I know that's not very right-on."
In October, Oldham midfielder Lee Croft was falsely accused of racially abusing a ball boy at Sheffield United, but the PFA's Taylor said he did not think there was a particular issue with the pitchside role.
"Ball boys are part and parcel of the game, to keep things moving. That didn't happen last night and the frustration erupted. I hope it's an isolated incident and we can move on," he added.
He said Hazard, who would normally receive a three-match ban for violent conduct, did not necessarily deserve a lengthy ban.
"He may well miss further games although I think he'll be sorry enough so there shouldn't be anything over and above what is expected for such an offence," he continued.
"I don't believe Eden Hazard had any intent to injure the ball boy whatsoever. He wanted to get the ball back and took the wrong action."
The incident between Hazard and the ball boy occurred with the scores 0-0 and Chelsea 2-0 behind on aggregate with 10 minutes remaining. The match finished 0-0 with the Swans securing a Wembley final date with Bradford.
Both the player and the ball boy apologised to each other after the match in the Chelsea changing room. Hazard, who said he was trying to kick the ball, and the 17-year-old were also interviewed by South Wales Police. The ball boy was interviewed by officers in the presence of his father and made no complaint about what happened.
Other Premier League players also reacted to the incident on Twitter.
Tottenham and Wales winger Gareth Bale was left mystified by referee Chris Foy's decision to show Hazard a red card.
"Unbelievable decision by the referee to send Hazard off but congrats to Swansea. Who'd have predicted this final?" he tweeted.
Manchester United defender Rio Ferdinand and Everton midfielder Phil Neville both agreed that Hazard deserved to be sent off.
Ferdinand tweeted: "I'm not the authorities, but in my eyes its a red for all those asking. Us players do put refs in bad spots at times unfortunately!"
Chelsea manager Rafael Benitez and Swansea counterpart Michael Laudrup agreed that both parties were at fault, although the Swans boss said that Hazard was initially at fault.
"They both talked. The boy knows he was wasting time. Hazard was frustrated and trying to get the ball back," Benitez said of the moment the two exchanged apologies in the changing room.
"We cannot change the situation. We are disappointed because we lost a player. The best thing for us is to move forward."
Laudrup added: "I think he [Hazard] will regret it when he sees it. The ball boy should have let it go but he was pushed first and then he kicks him.
"I understand as a player when you are behind and you are under pressure you sometimes say or do things you shouldn't but there are some things you cannot do."
Swansea City vice-chairman Leigh Dineen also criticised the Belgium international for "kicking out".
"I don't think it makes any difference whether someone is 17, 13 or 12," Dineen told BBC Radio 5 live. "You can't kick out at anybody."