Walcott, 21, is out of action following an ankle injury he sustained on international duty with England in midweek.
Although Nasri could return to face Owen Coyle's side, boss Arsene Wenger rued the loss of his pacey winger.
Cruel blow: Arsenal winger Walcott faces six weeks on the sidelines
'Samir could be involved against Bolton,' said Wenger. 'He has had a very good preparation and joins in training tomorrow. If needed I will integrate him in the squad for Bolton but for sure he will be back for our first Champions League game against Braga.'
Turning to Walcott's injury, Wenger added: 'The tackle looked quite bad. When you see how it turned his ankle I thought it would be three or four days or it would be four to six weeks. Unfortunately, it will be four to six weeks.
'We had some good news about the scan because at some stage we were scared it could be worse. But overall it is a question of patience and rehabilitation.'
Better news: Samir Nasri is set to return to action
Wenger will be also be without Robin van Persie for a similar length of time but the Gunners do have Johan Djourou and Denilson available after injury while new signing Sebastien Squillaci is on standby to make his debut. Midfielder Aaron Ramsey remains on course for a November return following the broken leg he sustained last season.
'Aaron is running,' Wenger revealed. 'He is in the normal process but he is a little bit more long term.'
The Gunners are still without striker Nicklas Bendtner, who is thought to be at least a month away from returning from a groin problem.
Meanwhile, Bolton boss Coyle is preparing to nullify the threat of Jack Wilshere. The 18-year-old has been an ever-present in the Gunners first team so far this season, and won his first senior cap for England in last month's friendly win over Hungary.
Coyle knows all about his ability following his loan spell at the Reebok last season. 'Jack is a very talented player. He is such a gifted lad and has matured no end,' said Coyle, who tried and failed to sign the midfielder on loan again this season.
'He came to us as a boy and went back to Arsenal as a man. He showed that by playing in the Arsenal first team and by turning out for England at full international level. There is no doubt he is going to have a glittering career.'
Arsenal winger Walcott joins the sick list with six-week lay-offSpurs can topple Arsenal, claims Rafael van der VaartJack's the lad to do us damage: Owen Coyle wary of Wilshere's threatSquillaci tells Wenger he is ready for Premier League bow against Bolton