Greenwood is rebuilding his career on-loan with Getafe this season after having assault charges dropped last year.
He qualifies for Jamaica through his parents and Hallgrimsson told the Athletic: "I really don't like to talk about 'what if', but of course it is in our thoughts.
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"I would love to have him on my team. Like all other coaches, I also want to have the best players available, but it is always up to the player himself.
"I just look at the person. I think what he's been through in the year and a half since it happened is worse than I can imagine, being in the press and on social media.
"I'm glad he's back because it's probably trauma. I give him my support at least."