As good as he’s been defensively this spring, Matt den Dekker realizes he won’t fulfill his dream of opening the season with the Mets unless he shows something offensively — and fast. “It’s got to come around,” said den Dekker, who has been perhaps the Mets’ most impressive rookie position player this spring. Den Dekker has made at least four highlight-reel catches in center field since the start of the Grapefruit League season, but as one team official said yesterday, “We already knew he could do that.” The implication was den Dekker’s .222 average — with 10 strikeouts in 27 at-bats — will spell an assignment at Triple-A Las Vegas to start the season. Defensively, he likely is ready for the major leagues. “From the first day I saw him he reminded me of [Jacoby] Ellsbury, body types, the way he can go get the ball,” Mets outfield coach Tom Goodwin said. “He has a stronger arm than Ellsbury has. Both of them can run, and he’s a difference-maker out there in center field.”