Parnell is 14 months out from Tommy John surgery on his right elbow and d'Arnaud has been on the disabled list since April 20 because of a fractured little finger on his right hand courtesy of a hit by pitch.
"I think we're pretty close with Bobby," Collins said. "I think he'll have one more rehab outing [at Double-A Binghamton] before he gets here."
The influx of returning talent should have a positive impact on the 31-27 team that left Sunday's action a half game ahead of the Nationals in the National League East. But Collins said just having these guys back on the active roster can't be an end in itself.
"It's got to be [a major impact], but they have to come up here and play well," Collins said. "We need performance. We don't need names. It think it's important that when [any of these guys] come back, they're ready to help us. We can't go through a two-week Spring Training with them. So we're not going to rush them by a day or two. They have to be ready to help us."
Save for one appearance, Parnell missed the entire 2014 season. He was the closer in 2013, saving 22 games in 49 outings. Since those heady days it's been one injury after another. His 2013 season ended in early September with surgery to repair a herniated disc in his neck. On the comeback trail after that he blew out his elbow and had surgery on April 8, 2014.
Collins said Parnell has been throwing in the low 90s during his Minor League rehab, but he thinks that will change when he gets back to the Mets. Parnell was a high-intensity closer with a 97-mph fastball.
"When you get under the bright lights the adrenaline is a little different," Collins said. "Certainly the sense of urgency's a little different, so we're hoping that the reports from the Minor Leagues will change and he'll be a little bit better when he gets up here. He could amp it up."
Asked about Parnell's bullpen role when he returns, Collins added: "You know what? I think I'll just get him in there. Let him get his feet wet again."
About d'Arnaud, Collins said he expected the catcher to join infielder Dilson Herrera at Triple-A Las Vegas on Monday. Herrera has been on the DL with a fractured tip of his right middle finger since May 15.
"I want them to get some consistent playing time so I think they're headed to Vegas today to get some at-bats there," Collins said. "I hope [d'Arnaud] will be back by the end of next week as well."