Praise for Carlos Tevez but Roberto Mancini sweats on Sergio Aguero's knee

Roberto Mancini conceded that he fears the worst over Sergio Aguero after the Manchester City striker was carried off on a stretcher yesterday with what looked like a serious injury to his right knee.

The City manager said he was "worried" about the fitness of his £40m striker, who will be taken for a scan on the injury, sustained in the eighth minute of the season's opening game, when the swelling around the critical area subsides. He fell awkwardly after a challenge from Southampton right-back Nathaniel Clyne, which was not a foul.

"We hope he will not have a serious problem," Mancini said. "It's his knee so I am worried, yes. Usually the knee is always dangerous. His knee injury was so strange [in the way that it happened]. I am worried. The good thing was that he tried to play but at the moment we can say nothing [definite].

"If it [turns out to] be a serious injury, normally we will change our [transfer window] plans, but for this we will have to wait. Maybe it will be one week or maybe it will be three or four weeks, or maybe after the international break."

Despite initially waving away the stretcher-bearers and walking to the side of the pitch on his own, Aguero eventually agreed to be carried down the tunnel. City won the game despite being 2-1 down to second-half goals from substitutes Rickie Lambert and Steven Davis after Carlos Tevez had first given them the lead. With David Silva also having missed a penalty in the first half, they recovered to win through Edin Dzeko and Samir Nasri's goals.

Mancini's suggestion that the potential absence of Aguero could change City's plans in the last two weeks of the transfer window is unlikely to make much difference to Adam Johnson's future. The club have received a £10m offer from Sunderland for the winger although it is expected that Tottenham Hotspur will run them close when it comes to signing Johnson.

There remains an interest in the Roma midfielder Daniele de Rossi from City. Even should Aguero's injury be serious, Mancini dismissed the possibility of a recall for Emmanuel Adebayor or Roque Santa Cruz.

On Johnson, Mancini said: "He is a young player, a young guy and wants to play. Now we wait for these 10 days. If we find a good solution and he can go to play, I think maybe it's better. If not, he will stay here. I don't know [about a loan]. We have two or three situations and are talking to him about this. He wants to play and I agree with him – when you are young you want to play."

Praising Tevez's contribution, Mancini said that it was "the first time in 10 years" that the striker had enjoyed a full pre-season. "When a player has had a good pre-season he can perform very quickly at the start of the season," Mancini said.

The Southampton manager, Nigel Adkins, who did not start with last season's top goalscorer in the Championship, Lambert, praised his players for coming within 20 minutes of beating the champions. "Our defenders have just played against the best forward line they've ever played [against] in their lives and I think manfully they've put a good performance in," he said.

"I think City only conceded 12 goals at home last season. We've just scored two against them. We knew it was going to be challenging but I thought we utilised the group of players we have intelligently to try and get something from the game.

"I give great credit to our players, they've put a shift in again. I think that's going to give us a lot of belief. We've got to be intelligent to give ourselves an opportunity to win because that's what ultimately it's about."