Mari shone in the FA Cup win at Portsmouth.
Speaking to Goal, Keown said Mari had the potential to become a key player in the team.
"He didn't have a lot to do apart from his distribution, but he won everything when necessary," Keown said.
"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.
"He wasn't rapid, but he 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."