Neal Maupay’s last-ditch tap-in scrambled Brighton a 1-1 Premier League draw at St Mary’s, with the French striker played onside by James Ward-Prowse.
The Saints skipper had raced back to cover McCarthy’s left post from Jakub Moder’s added-time free-kick, with the home keeper fearing he was not mobile enough to protect the entire goal.
A furious Hasenhuttl was left to lament a victory that slipped away in a manner he considered entirely avoidable.
The Saints boss insisted he would have replaced McCarthy with youngster Harry Lewis had he known about McCarthy’s injury.
“If you have a goalkeeper that is not 100 per cent fit in that moment and cannot jump and then you send someone back into the line, then the rebound that comes out is normally offside,” said Hasenhuttl.
“But Prowsey’s there, so he has a free shot at goal. This is something we cannot accept. There must be a message: ‘I cannot jump,’ and then we can make a change.
“We still had one change at the end and I could have changed the goalkeeper if necessary in this moment.
“But Alex must be definitely more professional than that. This is what I missed from him today in such a game.
“Yes it is hectic at the end and everything, but this is the reason why I sometimes keep the changes until later, because especially in such a game we’ve seen today, if you change too early you can be one man down.
“Today I had one change and I couldn’t take it because he didn’t say anything, and this for me is not acceptable.”
Armando Broja’s first-half strike had for so long left Southampton eyeing a victory to toast Hasenhuttl’s three-year anniversary at the St Mary’s helm.
But Maupay’s predatory finish plundered a last-gasp point for Brighton, who had been reduced to 10 men with Leandro Trossard injured and the visitors out of replacements.
Saints could now face a goalkeeping injury crisis, with 23-year-old Lewis the only fit senior stopper.
“Alex is out maybe for a long time, and Fraser Forster is also out injured,” said Hasenhuttl.
“It’s a situation where we have a fantastic game played, especially the first 45 minutes, dominated nearly the whole game, but we drew in the end and we feel we are not the winners today. This is awful and it’s hard to accept.
“It’s a hamstring injury, and this is the reason why we couldn’t play out any more in the second half.
“I didn’t know how tough it was, I thought he could only not play the long balls, so I kept him in the goal.
“But if I would do it again I would change him, but I couldn’t, because I didn’t have the message.
“There’s a reason why we have a reserve goalkeeper on the bench, this is no problem.
“Alex made a big save before, and he had kept us in with a chance of the three points at first.
“But when he then thinks he cannot jump into the corner for a free-kick, then I think it’s tough, it’s not possible.”
Brighton boss Graham Potter was left to hail a point clawed back at the last, and also to cross his fingers on Trossard’s injury.
The Seagulls had feared Trossard had dislocated his elbow, but later hoped the midfielder might only have suffered a minor nerve issue.
“We don’t think the injury’s as bad as first feared,” said Potter.
“We have belief as a team and the one thing I don’t doubt in my players is their character, spirit and willingness to go to the end.
“To go to 10 men for the last 10 minutes and come out with a point, it’s an incredible effort from the boys and it’s great to go home with something in the end.”