The Croatian international signed on a free transfer to provide squad cover for the injured Andy Carroll, and his new team-mate is convinced he can make a big impact
Petric, 32, signed for the Hammers on a free transfer on Tuesday after agreeing a deal that keeps him at the club until the end of the season.
He scored five goals in 23 Premier League appearances for Fulham last season, of which 14 were off the bench, and will provide cover in the absence of the injured Andy Carroll.
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And after taking part in Tuesday's training session with Petric, Jarvis likes the look of West Ham's new acquisition.
He told the club's official website: "Mladen is a good addition to the squad.
"That is what it is about, having a squad as when you have injuries and everything, others have to step up and play their part.
"For Andy, it is frustrating as you are working so hard to get over an injury and sometimes there are other niggles and things you pick up whilst you are doing this.
"Andy is a strong character and will continue to work as hard as he has been doing already to be back as soon as."
West Ham travel to Southampton on Sunday, hoping to rebound from a home loss to Stoke City in their last Premier League outing.
And Jarvis believes his team can claim a result on the road, insisting their defeat to Stoke was just a slight hiccup.
"They are a good team and pass the ball well, but we are in confident mood," he said.
"We were very disappointed in losing to Stoke in our last game, a result I think was a blip for us but something we want to get out of the system.
"We enjoyed a good point at St Mary's [Stadium] last season and then beat them at home, so we know what we can do, we will go there to get a result which will see us back on track."