Arsène Wenger remains "convinced" that Arsenal will qualify for the Champions League but he warned that there was no longer any margin for error after a Bacary Sagna blunder allowed Robin van Persie to mark his Emirates Stadium return with Manchester United's equaliser in Sunday's 1-1 draw.

Wenger spoke of the "disappointed love" that the home crowd had felt with regard to Van Persie, who received a pre-match guard of honour with his United team-mates from the Arsenal players to mark his club's 20th league title. His 43rd-minute goal, scored from the penalty spot after Sagna's poor pass and misjudged recovery tackle, was his 29th of the season. Van Persie later visited the Arsenal dressing room to see his old friends but Wenger's upset was located in him being "treated well by our defenders" on the field.

"I think that equaliser played a big part in this game because it put us in a difficult position," he said. "We had to open ourselves completely up and put everything forward and sometimes a little bit more with energy than with technique. We opened ourselves up for counterattack where we could have lost the game.

"Sagna is very disappointed. He is a player who gives absolutely everything in every game and I think he made a bad pass that can happen and maybe after, he panicked a bit because he maybe wanted to repair too quickly what he did. He had time to tackle but he has saved us so many times. It was unfortunate that it happened today."

Arsenal slipped to fourth place and, if Tottenham Hotspur were to beat Chelsea at Stamford Bridge on 8 May, Wenger's team could find they do not have control of their Champions League destiny. Before that, though, Chelsea must visit United. It is fiendishly tight.

"If we win our games, it will be enough," Wenger argued, "and I'm convinced we will be there. There is no room for dropping points but the other teams are in the same position. We know we cannot drop points."

Wenger took issue with the notion that it had been inevitable that Van Persie would score and he suggested that the value of penalties did not weigh as heavily. But he was pleased, in general, with the respect that was shown to his former player.

"I don't think he scored," Wenger said. "He scored a penalty. It counts but it's not completely the same. When you buy a goalscorer, you always look at how many goals he has scored with penalties and how many he scored in open play. But he took his penalty very well. I know when he decided to take the penalty that it was 80% he scores. He was our penalty taker. He does that very well. He does a lot of things very well.

"He was in our dressing room [after the match] because the food is better in our dressing room than their dressing room. We are rivals [with United] but at the end of the day, we decided to sell him. You want us to make a guard of honour and to respect the champions but you have to accept as well that respect exists after the game. You want to give absolutely everything for the 90 minutes and, after the game, you can still talk to each other."

United now know that they cannot reach a record 96-point Premier League finish and Sir Alex Ferguson lamented missed chances and the referee Phil Dowd's decisions to book five of his players. "When we had a foul, he's booked us right away," Ferguson said. "We have one of the best disciplinary records in the country. For Manchester United to have five players booked, I don't think that's right. Arsenal set off at a fantastic pace, very aggressive, with a lot of tackles and the referee just couldn't keep up."