The Gunners signed the left-back for an estimated £8 million on transfer deadline day to cover for the injured Kieran Gibbs, and the Chilean bemoaned the exit of the talented defender.
"Monreal's exit is a very sad moment for the team. It deprives us of a very important player who performed to the highest standards," a frustrated Pellegrini said in a press conference.
The coach went on to state that not only was it a loss in footballing terms but it also epitomises the dire financial state that the Primera Division club find themselves in.
"Unfortunately, the reality of the club is what it is, and there is no way of hiding it. It was an option that came up at the last minute, and the club thought it was appropriate," he added.
"I understand they have to try and balance the books, because the pressure is on us from all sides, and we have received punishments. It is a major loss to us in sporting terms."The move comes less than a year after Arsenal swooped for Malaga's star midfielder, Santi Cazorla, and Premier League clubs - most notably Manchester city - are also reportedly keen on talented attacker Isco.