Roberto Mancini will anxiously await the scan results on Sergio Agüero after the striker was carried off on a stretcher with a knee injury early in Manchester City's 3-2 win over Southampton on Sunday. The Italian confessed himself "worried" at the potential long-term loss of Agüero, who scored 30 times in his debut season, with City certain to enter the transfer market for a replacement if this proves to be the case.

Mancini said: "We don't know at the moment, we need to wait maybe two days for the scans. We hope it's not serious but at this stage I don't know. I am worried, yes. This can happen in every game, it is clear that it is important that you are also lucky through the year but today it's a problem. Usually the knee is always dangerous, this injury was maybe so strange. If it is a serious injury, we will probably change our plans [to bring in a replacement]. Maybe he is out for a week, or three or four weeks, or after the international break."

Agüero gave his manager some comfort with a Twitter message last night suggesting that there was "no significant injury" although this may be wishful thinking. He suffered the injury after eight minutes from a Nathaniel Clyne tackle that Mancini had no issue with. If Agüero is ruled out in the long term, Mancini said there would be no chance of Emmanuel Adebayor or Roque Santa Cruz replacing him. "At this moment definitely not," he said.

City's opening match in defence of their first title since 1968 nearly ended in defeat. After David Silva's early penalty miss, Carlos Tevez's first-half opener was cancelled out by goals from Rickie Lambert and Steve Davis after the break before Edin Dzeko and Samir Nasri secured the points for City.

"We had [a difficult] 20 minutes to the end," Mancini said. "But when you play at the start of the season, it is always difficult and also because we had 15 players away with the national team, so we only had all the players on Saturday. We didn't play well with our defenders but it is not always their fault. We play very well but, when you have so many chances, we should have closed the game. After the second half I think we played so for many reasons and we did some mistakes. We were not 100%."

Nasri added: "We didn't panic. We were a little bit low but we said we can do it, we did it last year for the crucial game [3-2 on the final day to win the championship] but next time it will be better if we win one or two nil. Southampton played really good football. It reminds me a little bit of Swansea last year, it was difficult but at the end we got three points.

"Sometimes it's a little bit difficult when you miss the first chance of the game, it can make the confidence a little bit low and they punished us. But I thought we showed great character."

Mancini handed Jack Rodwell a debut following his £15m move from Everton, preferring the 21-year-old over Nigel de Jong. "Nigel played on Sunday [in the Community Shield] and in midweek, so I wanted to see Jack and give him experience," the Italian said.

Rodwell's mistake led to Davis's goal but Mancini said: "We were so many players in attack and when you attack you can concede also."

Mancini would not confirm that De Jong may be sold to Internazionale and, regarding a possible move for Roma's Daniele De Rossi, he joked: "You need to speak with Mr [Brian] Marwood."

While Marwood is the club's football administrator Mancini confirmed that Adam Johnson may leave before the transfer window closes on 1 September. "Adam is a young guy, a good player and wants to play. If we find a good solution for him then I think maybe it's better but [otherwise] he stays here," the manager said. Whether Johnson leaves on loan or permanently is to be decided.

For Nigel Adkins, the Southampton manager, there was disappointment on his club's return to the Premier League after seven years. "The game plan was to be in with a chance of getting something from the game. We ran them close but our endeavour is to win – we've got to find ways to win," he said.

Lambert said of his finish: "You could see on my face how much the goal meant to me. I've been thinking about it for a long time and I couldn't have picked a better moment to score my first Premier League goal. We're proud of our efforts but we are devastated we haven't got anything from the game. We knew we had a hard start to the season but we came here to compete, not just to survive, and today we showed everyone how good we can be."