Walcott is in danger of being reduced to a fringe figure following a disappointing campaign that ended with the England winger being left out of Arsenal’s FA Cup final win against Chelsea in May.
As if that wasn’t depressing enough for Walcott, he has watched Wenger splash out a club record £52 million ($68 million) for Lyon striker Lacazette and make a so far unsuccessful attempt to sign Monaco winger Thomas Lemar.
But, in his 11th year with Arsenal, 28-year-old Walcott served notice he isn’t going to be cast aside without a fight as he produced two clinical finishes and laid on the cross for a Lisandro Lopez own goal.
Olivier Giroud and Alex Iwobi were also on target as Arsenal ran riot in their first match in the four-team friendly tournament at the Emirates Stadium.
Lacazette, introduced to a huge roar, had few chances to impress in his brief cameo, but the France international is likely to start against Sevilla on Sunday.
Franco Cervi and Eduardo Salvio were the scorers for Portuguese champions Benfica, who looked lost at the back following the sale of Sweden defender Victor Lindelof to Manchester United.
Walcott started for Arsenal with Lacazette on the bench and wantaway forward Alexis Sanchez not due to return to training until Sunday after Confederations Cup duty with Chile.
But Walcott’s headline-grabbing show came after Arsenal fell behind in the 11th minute as Jonas set up Cervi to drill a low strike past David Ospina.
Sead Kolasinac, a free transfer recruit from Schalke, made his reputation as a buccaneering left-back and, although the Bosnian was deployed in a back three by Wenger, he got forward to help Arsenal equalise in the 24th minute.
Pressuring Pizzi into surrendering possession as the Benfica defender tried to shepherd the ball out of play inside his own area, Kolasinac saw goalkeeper Julio Cesar was out of position and chipped a cross towards Walcott, who volleyed home from 10 yards.
Walcott bagged his second in the 32nd minute, converting a Francis Coquelin cross from close range.
Coquelin then limped off having been caught on the ankle earlier in the half and Benfica equalised in the 39th minute.
Ainsley Maitland-Niles squandered possession outside the Arsenal penalty area and Pizzi pounced to set up Salvio, whose deflected effort beat Ospina.
But Wenger’s side regained the lead in the 52nd minute when Walcott’s cross towards Giroud was deflected into his own net by the lunging Lisandro.
Giroud then netted Arsenal’s fourth with a volleyed finish from promising youngster Reiss Nelson’s pin-point cross and Iwobi made it five with a ferocious strike into the roof of the net in the 70th minute.
In Saturday’s other Emirates Cup match, Sevilla beat German Bundesliga runners-up RB Leipzig 1-0 thanks to Wissam Ben Yedder’s 34th-minute penalty.