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If pro baseball player Matt Kemp has his way, Rihanna will be the "Only Girl" in his life -- permanently!

A source tells the new issue of Us Weekly that the L.A. Dodgers center fielder is close to popping the question to the 22-year-old singer.

"They are in love," says a source. "Seeing them interact, it's definitely possible he could propose -- but it'd have to be a long engagement, because they're both so focused on their careers."

What makes their eight-month relationship -- which often requires them to spend several days apart -- work so well?

"He centers her, as he has a more normal life," a pal close to Rihanna tells Us of Kemp, 26. "He allows her space, but they text and talk all the time. And they really have fun like kids when they're together. They like to laugh and be silly."