The world number five needed a lengthy injury timeout in the Brisbane International men's singles final after drawing level in the second set against Bulgarian Grigor Dimitrov.
Nishikori went on to lose the deciding set, suffering a first defeat to Dimitrov in four meetings.
"I hurt it during the match yesterday in the final. It's not too bad but obviously I needed some rest," the 2014 U.S. Open runner-up said.
The 27-year-old, who beat Stan Wawrinka in the Brisbane semi-finals, hopes to recover in time for Melbourne Park, where the year's first grand slam begins on Monday.
"It was a really tough tournament," Nishikori said. "There were five of the top 10 in the tournament.
"I played Dimi in the final and it was another tough match and I used a lot of energy on the court last week."
(Reporting by Sudipto Ganguly in Mumbai; editing by Peter Rutherford)
