Striker Robin Van Persie wanted to leave Arsenal and left the club with no choice but to sell him, manager Arsene Wenger said on Friday after the Dutch international joined Manchester United.
Last season's Premier League top scorer signed a four-year deal at Old Trafford, ending an eight-year stay with Arsenal for whom he scored 132 goals in 278 appearances.
"We regret that we have lost a great player and for one simple reason - he only had one year left on his contract. He wanted to go," Wenger told a news conference on Friday.
"We anticipated this situation might happen, therefore we bought Lukas Podolski and Olivier Giroud.
"(Selling to a rival) is not what we wanted but there was not much choice. We had only one club really interested at the level we thought would be acceptable and that was Manchester United.
"At the end of the day, was the only decision we could make."
British media reported the deal to be worth 24 million pounds and Wenger said Arsenal had already "invested the money" with the signings of Spain international midfielder Santi Cazorla from Malaga, German international winger Podolski from Cologne and France striker Giroud from Ligue 1 champions Montpellier.
"If needed we will invest more, but if you look at the number of players we have, we have a massive squad. We have plenty of players who have no chance to play at all.
"At the moment I believe we have what is needed to be ambitious and to play our game."
Arsenal, third in the league last season - 19 points behind the Manchester clubs - begin their campaign with a home clash with Sunderland on Saturday (1400).
Winger Alex Oxlade-Chamberlain (ankle) and defender Laurent Koscielny (calf) miss the game but forward Theo Walcott, who pulled out of England's friendly with Italy in midweek because of a bruised thigh, has recovered.
Defender Thomas Vermaelen will replace Van Persie as Arsenal captain.
Meanwhile, Van Persie, who scored 37 goals for Arsenal in all competitions last season, is set to make his debut in United's Premier League opener against Everton on Monday.
"Robin is a world-class striker with a proven record in England and in European football," United manager Alex Ferguson told the club's website (www.manutd.com).
"His talents need no introduction to our fans - he has scored a number of goals against us in some classic battles with his former club. His movement, finishing and all-round ability are outstanding."
British media reported that United paid 24 million pounds ($37.78 million) for the 29-year-old.
"It's an honour to sign for Manchester United. I am looking forward to following in the footsteps of so many great strikers, bringing my experience and playing my part to help the team compete for the biggest trophies in the game," Van Persie said. "I can't wait to get started."
Van Persie had one year remaining on his deal with Arsenal, who he joined in 2004.
He said in July that he would not be extending his contract with the Londoners because he was unhappy with the direction they were taking.
Arsenal, who finished third in the Premier League last season, have not won a major trophy since the 2005 FA Cup.