Sanchez failed to inform UK Anti-Doping of his whereabouts on Monday while he was finalising his switch to United.
Injury-plagued midfielder Santi Cazorla has not played for Arsenal since October 2016 and his current deal is due to expire in the summer.
Arsene Wenger will allow Olivier Giroud to leave Arsenal this month if the club can sign a replacement.
Aubameyang will be part of the Dortmund squad for their game against Freiburg on Saturday (27 January) after Arsenal and Dortmund fail to agree fee.
Arsenal had a difficult start to 2018 and have won just two of their five games this month which include losses to Bournemouth and Nottingham Forest in the FA Cup.
Arsenal are keen to sign Aubameyang before the end of the transfer window but will have to meet Dortmund's valuation said to be around £55m.
Arsene Wenger has claimed that Arsenal are "strong enough" even if they fail to sign Pierre-Emerick Aubameyang from Borussia Dortmund this month.
Borussia Dortmund boss Peter Stoger is eager for the club to find a solution over Pierre-Emerick Aubameyang's proposed move to Arsenal.
Arsene Wenger has revealed that he is displeased at the media for always putting Arsenal down despite their recent success, winning three FA Cups in the last four years.
Arsene Wenger admits his new Arsenal signing Henrikh Mkhitaryan may have some confidence issues after struggling at Manchester United.
Sanchez, 29, joined United on a four-and-a-half-year contract, with Henrikh Mkhitaryan moving in the opposite direction.
Arsenal want to sign Jonny Evans to strengthen the back line in January as they continue holding talks with Dortmund for Aubameyang.
Nacho Monreal was rated as a doubt by Arsene Wenger earlier, but the defender has recovered from a minor knock and will be available against Chelsea.
Arsenal hopeful Henrikh Mkhitaryan and Aubameyang arrival will convince Ozil to extend Emirates stay
Mesut Ozil has less than six months left on his contract at Arsenal and is attracting interest from Manchester United, Inter Milan and Juventus.
Pierre-Emerick Aubameyang played alongside Henrikh Mkhitaryan when the latter was at Borussia Dortmund. The duo were impressive while leading Dortmund's attack.
New arrival Henrikh Mkhitaryan and Mesut Ozil can successfully play in the same team together at Arsenal, according to manager Arsene Wenger.
Arsene Wenger claims that a deal with Borussia Dortmund for striker Pierre-Emerick Aubameyang is not close at this stage and insists Arsenal have "other things in mind as well".
Aaron Ramsey could return for Arsenal in Wednesday's Carabao Cup semi-final second-leg tie against Chelsea after illness, but Nacho Monreal looks likely to miss out.
Arsenal ready to sanction Kelechi Nwakali's move to MVV Maastricht once they terminate VVV Venlo loan
Arsenal are keen to terminate Kelechi Nwakali's loan deal with VVV Venlo and send him on a temporary basis to Dutch second-tier club Maastricht.
Alexis Sanchez became Manchester United's first signing of the January transfer window. The Chilean international has been handed the iconic No 7 shirt at Old Trafford.
Sanchez has signed a four-and-a-half year contract at Old Trafford worth reportedly around £500,000-a-week while Mkhitaryan has penned a long-term contract with Wenger's side.
Sanchez had entered the final six months of his contract and refused to sign an extension as the Gunners were unable to meet his monetary demands.
Arsenal transfer rumour: Gunners agree personal terms with Pierre Emerick-Aubameyang and Henrikh Mkhitaryan
Pierre Emerick-Aubameyang and Henrikh Mkhitaryan played together at Borussia Dortmund and impressed together at Signal Iduna Park.
Pierre-Emerick Aubameyang remains Arsenal's preferred target to replace Manchester United bound Alexis Sanchez.
Aubameyang's transfer to north London is seemingly imminent while reports claimed that Arsenal had beaten Tottenham Hotspur to Malcom's signature earlier this week.
Arsenal transfer news: Alan Smith believes Mesut Ozil is close to signing a new contract with Gunners
Ozil has only six months left on his current deal and has been linked with a move out of the club in the summer.
Arsenal could use Olivier Giroud as part of player-plus-cash deal for Dortmund's Pierre-Emerick Aubameyang
With Alexis Sanchez and Olivier Giroud's potential exits from Arsenal, Pierre-Emerick Aubameyang's addition will bolster their strike force.
Arsenal planning to recall Kelechi Nwakali from VVV Venlo loan spell, Porto interested in temporary deal
Kelechi Nwakali was expected to spend the entire campaign with VVV Venlo but, Arsenal are planning to recall him owing to limited game time.
Arsenal manager Arsene Wenger has addressed speculation over deals for Pierre-Emerick Aubameyang, Malcom and Jonny Evans after confirming that a Sanchez-Mkhitaryan swap is likely.
Arsenal's Arsene Wenger believes the part-exchange deal with Manchester United that would see Alexis Sanchez and Henrikh Mkhitaryan trade clubs should happen in the next 24 hours.