The Gunners go into the North London derby against Tottenham with skipper Martin Odegaard ruled out with an ankle injury.
Walcott told football.london: “So this is the journey, and this is the test Mikel hasn't really faced as an Arsenal manager having a lot of players out at the same time. He has had players out, of course, but players, you would say, haven't been starters.
“I suppose, you know, maybe Partey is the exception, he’s probably one of them. But this will be the biggest test for Mikel as a manager to see if his squad is good enough to win this league because that's the difference with Man City.
“Their squad is… the bench is good enough to win the league. So this is where Mikel is going to have to be tested.
“This is where players themselves that are waiting for opportunities, saying ‘They're gonna be that player, they want to play that game, I want to be in the starting line-up.
“There's an expectation for every player to play at Arsenal, I get that. But I just feel like now this is the biggest test and it's nice to have it right at the start of the season when the games are a little bit more staggered.
“It gives a bit more opportunity for these players to come back, but it also gives an opportunity for the players to put a statement and say, ‘Look, I want to challenge Martin Odegaard. I know he's the captain but I want to challenge him right now.’
“I think that's the positive side of Mikel, seeing his training particularly, he challenges every player. He gets them to ask questions about themselves, to take responsibility consistently and to have connections.
“And I think when you have that in a squad environment, you're not going to get cliques and that's the thing where if a player does come in and do exceptionally well and Martin comes back in, it's not going be personal.
“That's the side of it where I've experienced, definitely you know, if players come on and they’ve done well, they expect them to start next game and then they're on the bench and then suddenly they're throwing the toys at the pram… It's a team game. “