Gunners boss confident that wantaway Chilean will soon be back to his best.
Premier League title favourites failed in attempts to sign Sanchez on deadline day.
Gunners boss believes rising transfer costs have become "completely disconnected to reality and the truth".
Arsene Wenger's side are beginning Europe's secondary club competition for the first time in two decades.
The 25-year-old has not played this season amid ongoing speculation over his future.
Josep Maria Bartomeu says that the official announcement should be done within a month.
Sanchez was looking to leave Arsenal as Manchester City were keen on signing him.
The 17-year-old left Pep Guardiola's side to join Borussia Dortmund.
Sancho joined Borussia Dortmund and signed a long-term deal with the Bundesliga club.
Spaniard has agreed on a two-year deal, with the option of a further season.
Manchester City had agreed a £60m deal to sign the Chile international on deadline day.
Manchester City are expected to come back with an improved offer for Sanchez.
Manchester City outcast now on Frank de Boer's radar after three failed bids for celebrated former loanee.
Manchester City are likely to make a fresh offer to sign Sanchez on deadline day.
Manchester City manager Pep Guardiola is keen on bringing Sanchez to the Etihad.
Former City 'keeper Given thinks Sterling could eventually become a £100m player.
City have been pursuing Sanchez for the entirety of the summer.
Redknapp says Arsene Wenger's Arsenal need 'a couple of powerhouses'.
Delph has had limited opportunities at the Etihad and could leave before the end of the transfer window.
Pep Guardiola dropped Aguero from City's starting XI in their 2-1 win over Bournemouth.
Sanchez has one year left on his contract at Arsenal and has refused to sign a new deal.
City remain interested in a cash-only deal for Sanchez.