Guimaraes was forced off just before halftime in their Champions League win against PSV Eindhoven last night.
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Howe said post-match: "He's a little bit sore and there's a little bit of swelling around his ankle.
"We hope it's not serious but it's difficult to know. It's a worry that he's had to come off."
Newcastle won 3-0 on the night through goals from Yoane Wissa, Anthony Gordon and Harvey Barnes.
