The Spanish centre-half started alongside David Luiz in the 2-0 victory.
An incredible stopper in his time, Keown knows a thing or two about defending. And he liked what he saw from the Gunners' newboy.
"He didn't have a lot to do apart from his distribution," he told Goal. "But he won everything when necessary.
"You could see that he was a little bit short of fitness and that they are building him up. He looks to have the basis of a good player there.
"There were important first steps on Monday night, even though he wasn't tested that much because in the second half Arsenal completely dominated possession - it was a bit of a football lesson for Portsmouth."
Keown added: "I think he looked very solid, he has the height in inches. You look at the stature of the player and he won everything in the air.
"He wasn't rapid, but was quick enough to deal with all the situations.
"We were playing against a League One side who had a lone striker so he and David Luiz were able to boss him between the two of them.
"I think it was important that he had Luiz alongside him to talk him through the game. Now we have to give him time, he has to be given more opportunities."