Manchester City boss
Aguero fell awkwardly after an early challenge by
It was quickly obvious the
Mancini confirmed it will be 48 hours before City can make a proper diagnosis.
However, as with all such injuries, the fear Aguero may have damaged his cruciate ligaments is obvious.
"I am worried," said Mancini.
"We don't know how serious it is because we need to wait maybe two days for the scan.
"But usually the knee is dangerous.
"The injury was so strange because it was not a bad tackle. It was not strong.
"The good thing was that he wanted to try and play. But at the moment we can't say anything."
Mancini conceded that if the worst is confirmed later this week and Aguero is ruled out for an extended period, he may have to change his squad building plans.
He is still hoping to bring in a new face, with strong rumours linking him with Roma midfielder
"If it is a serious injury we will probably change our plans," said Mancini.
"But we need to wait. Maybe it will be one week or three or four weeks, after the international break."
Mancini also confirmed the complete absence of
Johnson was at the
However, he has become tired of being a bit-part player with the Blues, although Mancini insisted he has no axe to grind against Johnson, who started England's midweek win over
"Adam is a young guy," said Mancini.
"He is a good player and he wants to play.
"In these 10 days, if we find a good solution for him and he can go to play, maybe it is better.
"I don't know about a loan. We have two or three situations. We are talking with him about this.
"But if he stays here I don't have a problem."
It formed additional talking points from yet another 3-2 win, which matched exactly both the scoring sequence and ends the goals were scored in from that momentous title-winning triumph over Queens Park Rangers in May.
The only difference was that on this occasion, City also missed a penalty, a woeful attempt by David Silva when the game was still goalless.
Carlos Tevez broke the deadlock just before the break, then Southampton substitutes Rickie Lambert and Steve Davis hit back to threaten a shock win for the newly-promoted outfit.
However, the lead lasted just four minutes as Edin Dzeko levelled - just as he did against QPR - before Samir Nasri drove home the winner 10 minutes from time.
"We didn't play well and we didn't defend well," said Mancini.
"But it is not always their fault.
"When a team like us has 10 chances to score, you should close the game.
"The second half I think we played so-so for many reasons and the players were so-so."
Saints boss Nigel Adkins had sprung a surprise by leaving Lambert out of his starting line-up after scoring 27 goals in the Championship last term.
However, he felt it was a better use of Lambert's abilities to have him fresh for the latter stages.
"I would have thought he was upset because everyone wants to play," he said.
"But I have to make decisions.
"We came up with a game plan and he was on the pitch to score when it mattered.
"We expected the opposition to have a fair bit of the ball, so instead of Rickie running round and being shattered, he was on the pitch, scoring a goal and being a threat."
Adkins also confirmed he expected more signings before the 31 August deadline, although offered no update on the deal to get Gaston Ramirez to the club, amid reports the transfer is close to collapse.
"We have been linked with a few players," he said.
"I don't know if there are any more developments but there will be more players coming into the club before the transfer window shuts."
Aguero claims his injury is not a serious one.
He wrote on his official Twitter account: "The studies I had done revealed there's no significant injury but it was a very tough blow to my knee.
"Tomorrow, the physicians will evaluate the time it will take for me to recover.
"I have no words to thank for your concern for me and with this much support, I'll recover faster.
"And I want to congratulate my team mates for their effort in turning this game around. Now more than ever - C´MON @MCFC !!"
English Premier League