"He might be out for a while," Belichick told reporters on a conference call.
Belichick stopped short of saying Fletcher would be placed on season-ending injured reserve, but things seem headed in that direction, especially with the normally guarded Belichick providing an unexpected injury update.
"It's unfortunate because he's worked hard. He had a really good offseason and training camp," Belichick said of Fletcher, a third-year player from Montana State. "We have good competition at that position and it will be an opportunity for somebody else to step in, but it's unfortunate for Dane."
Multiple reports indicated that Fletcher tore his ACL while covering a punt in the first quarter of the team's 7-6 win over the New Orleans Saints.
The 6-foot-2, 245-pound Fletcher played 22 percent of the defensive snaps in 2011, although that number likely would have been higher had he not missed five games with a thumb injury. He started at middle linebacker in Thursday's preseason opener, in place of the injured Brandon Spikes, and appeared primed to be a top backup at linebacker in addition to a front-line special teams contributor in 2012.
Bobby Carpenter, a seven-year veteran who joined the Patriots this offseason after stints with the Cowboys, Dolphins and Lions, is a likely option to see his role expand because of Fletcher's injury.
"He's a very experienced player and he's played inside and outside in the 3-4 and he's played different positions in a 4-3 and nickel defenses. He has a lot of experience in the kicking game," Belichick said of Carpenter (6-2, 250), who entered the NFL in 2006 as a first-round draft choice. "So he's done a lot of different things. He's a versatile player with a lot of experience."
First-year linebacker Mike Rivera (6-2, 255) was also thrust into some early action in Thursday's preseason opener, and could potentially carve out a role for himself after spending last season on the team's practice squad. The former Kansas product also showed up on two first-string special teams units Thursday, a sign that he could be rising in the eyes of the coaching staff.
"He had a good year on the practice squad and did enough things that we re-signed him this season. He's been very competitive in camp, so we're trying to give him some opportunities to see what he can do," Belichick said of Rivera. "He's certainly well ahead of where he was last year, because of the spring practices and [organized team activities]. He's a hard-working kid who has shown up in the kicking game and on defense, and has done some good things."