Mikhail Gordeychuk made it 4-2, but Arsenal's job was done.
Arsenal moved to six points and are among six clubs with two wins from their opening games - Swedish surprise package Ostersund in Group J; Steaua Bucharest in Group G; Lazio and Nice in Group K and Zenit St Petersburg in Group L. BATE's goalkeeper, Denis Scherbitski, passed the ball straight to Walcott who couldn't believe his luck and scored easily to double Arsenal's lead.
Arsene Wenger made nine changes to a winning team but even if his gamble was rewarded with victory, this was not the one-sided win the scoreline suggested.
He also urged their hosts' supporters to avoid a repeat of the racist abuse that has previously undermined European fixtures, and revealed plans to meet Belarusian former Arsenal midfielder Alexander Hleb, who started his career at BATE.
"As a forward you want to score as many goals as you can, there was a chance for me to take the penalty but Olivier was on 99 goals", he said.
Mertesacker is to retire at the season's end to become the club's academy manager, and he similarly recognises the importance of those younger players gaining first-team experience. However, performances like that against Borisov will do his confidence a world of good and show that he still has it in him to perform at the greatest stage of them all. You forget, last year I was at Bournemouth, the year before that I was injured, so it has been a while since I felt like a proper Arsenal player. He's on the way up. "He is on the way up and he has only been stopped by a series of injuries", Wenger added. Hopefully, I just pray, he is not hampered by any more problems and that will see him getting stronger and stronger.
Wenger's side beat Cologne 3-1 at home in their first outing a fortnight ago and the German side lost again on Thursday, going down 1-0 at home to Red Star Belgrade. Wilshere and Giroud played a clever one-two, and although Walcott's header from Wilshere's cross was well saved by Denis Scherbitski, the rebound fell kindly for him to poke home.