The 32-year-old Senegal defender signed for Newcastle on Thursday until the end of the season.
Diatta became a free agent earlier this season after buying himself out of his Besiktas.
Keegan: "It was common sense - like an insurance policy.
"He'll benefit us from the fact that he's a centre-half, and an international player. I've been very impressed with his attitude from what I've seen around the training ground.
"He's come over in a very honest and determined way to earn a contract, and after four days I felt it was the sensible thing to do.
"He might take a week to get fit, and there's also a language problem. He speaks a little English, but isn't fluent."