The Brazilian scored his first Gunners goal for 15 months in Arsenal's 3-0 away victory over the Blades on Sunday.
"There's been a lot of talk about Arsenal's recruitment and have they spent too much money for certain individuals, but this guy (Martinelli) - it was a snip they paid for him," Keown said on BT Sport.
"(The goal) really will help him. He's been through a lot, 15 months since he last scored, he's had injuries, his ACL.
"But he's an instinctive finisher, he knows where to run."
Keown added: "I've almost felt like I've been signing a petition to get him (Martinelli) in the team. He's a great talent.
"We have to look at it and say ok, has there been any underlying injury? Are there other reasons why he's not been in the team? But now he's in the team, I think he vindicates that selection and he should stay in the team. I don't think you can take him out now."
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