The 24-year-old defender joined the Gunners on a free transfer in June after ending a five-year spell at Schalke 04.
The Bosnian international impressed during Sunday's Community Shield against Chelsea after coming on as a substitute when Per Mertesacker suffered a head injury.
Kolasinac ended up scoring an equaliser to force the tie into penalties, which were won 4-1 by Arsenal.
"Sead looks an absolute tank. He's one tough unit to get past and I'm sure his physical attributes are going to be key to us this season," FourFourTwo quotes Walcott as saying.
"Honestly, he is an absolute animal. He's incredible. I see him in the gym every day; you can just tell he looks after himself well, he eats the right food and he's professional. You can see the power he's got when he's got the ball, his pace and everything. It's beautiful to watch and he's a great addition to the squad.
"Trust me, he's not easy to get around - he's quite wide as well. He fits perfectly for the Premier League, he's absolutely perfect. He's going to show something different. I've seen glimpses of him in pre-season and he's going to get fitter, stronger and quicker. This league is the best and I think he could be one of the best in the league."
Arsenal's only other signing of the window so far is club-record £52m striker Alexandre Lacazette.