Matsuzaka tossed six innings for Double-A Binghamton on Saturday, allowing one run and three hits with three strikeouts and no walks.
"We're very happy with the way he threw," Collins said. "But, right now, we aren't sure where the fit is going to be when he's ready to pitch here."
Jacob deGrom's impending return from the the disabled list means there won't be a spot in the rotation for Matsuzaka, who pitched three scoreless innings for Class A St. Lucie last Monday.
In the bullpen, righthander Buddy Carlyle is establishing himself, further limiting the need for Matsuzaka. Carlyle, who has been designated for assignment twice this season, has an 11 1/3-inning scoreless streak with a 0.55 ERA and 0.74 WHIP. He pitched the ninth inning of Saturday's win over the Cubs and picked up his first major league hold since 2009 last Monday against the Phillies.
"Buddy has pitched very, very well," Collins told MLB.com. "When it comes to the roster spaces and who's on or who's off, [general manger Sandy Alderson is] pretty heavily involved with that. When [Matsuzaka is] ready to go, we'll have to figure out where he fits. Right now, we haven't really had that discussion."
The Mets could leave Matsuzaka on the DL until Sept. 1, which could mean a fourth rehab outing for him in the minors.