Niese underwent a battery of tests this week at Henry Ford Hospital in Detroit after experiencing a rapid heartbeat during the sixth inning of his last start, Saturday at Texas.
Niese completed his final test Wednesday afternoon, when the team returned a heart monitor for diagnosis that the left-hander wore for 24 hours.
"He's going to be fine," Collins said.
Collins also said he talked to third baseman David Wright, who's out with a stress fracture in his back. Wright has swung in a pool and could be ready for rehab games in about a week.
Adam Rubin covers the Mets for ESPNNewYork.com. Information from The Associated Press was used in this report.