The Rams midfielder, whose first season at Pride Park was hampered by injury, is set for a spell on the sidelines for 'some time'.
The 24-year-old, whose maiden season at Pride Park was hampered by injury, appeared to be back on track this season, with six appearances including three Championship starts under his belt (Transfermarkt).
However, the 2018 signing from Reading, who can play central midfield or centre back, will be out for "some time" with a muscle injury, according to Cocu, who admitted it "came out of nowhere".
"Evans is out for some time with a muscle injury - it will take some time," Cocu, speaking at his pre-match conference ahead of Friday's Championship clash with Cardiff City at Pride Park, is quoted by the Derby Telegraph as saying.
Asked whether it be a matter of months out, Cocu replied: "I always have to be careful with muscle injuries. It came out of nowhere. We thought it was a minor issue, but when we examined him a little bit better in the hospital it came out it was more serious than we thought. So it is a big disappointment and he has to work hard to come back."
Following his Rams debut in the Carabao Cup trip to Oldham Athletic last August, Evans was forced off in the first half of the Championship clash with Ipswich Town a week later, spending time on the sidelines until just after Christmas.
Then, from mid-February to late April, he only played 45 minutes in the Championship, in the loss to Aston Villa, before featuring towards the tail end of the season and earning a spot on the bench in the playoffs (Transfermarkt).
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