Phillips likely will miss next Sunday's game against the Cleveland Browns and may be sidelined a few weeks.
"Right now, it is an MCL and will end up being a week-to-week thing," Coughlin said.
Coughlin acknowledged that the injury is similar to the one that starting right tackle David Diehl suffered against Tampa Bay on Sept. 16. Diehl has missed the last two games due to the injury.
Coughlin also did not have any encouraging news regarding wide receiver Hakeem Nicks, who missed Sunday night's loss to the Philadelphia Eagles with swelling in his knee and pain in his surgically repaired foot.
Nicks has not played since the win over Tampa Bay. He was held out of the Giants' ensuing game, a win at Carolina, because of the quick turnaround for that Thursday night game.
But after taking time off and experiencing swelling in the knee last Thursday, Nicks was downgraded to out on Saturday.
Coughlin admitted he was concerned about Nicks' health going forward this season and stated he was unsure of the star receiver's status against Cleveland.
"How much concern can I show you?" Coughlin asked. "I won't know until I see what happens, where he's at as they go forward and they analyze him and the doctors get their hands on him."
Coughlin said Nicks' knee issue, something that first popped up when he banged his knee late in the fourth quarter against Tampa Bay, flared up after he ran last week following the extended rest he had.
"It's a swollen knee," said Coughlin, who added that he did not know whether Nicks improved over the weekend.
The Giants did get safety Tyler Sash back on Monday from suspension for violating the NFL's performance enhancing substance rules.
New York can use Sash since Phillips and other secondary players are banged up, including cornerback Jayron Hosley (hamstring), safety Antrel Rolle (knee), cornerback Michael Coe (hamstring) and cornerback Corey Webster (broken bone in hand).
Phillips is a key component in the defense and had been playing well and looking more like the player he was prior to undergoing microfracture surgery on his left knee in 2009.
"Naturally, (Phillips) is upset about it," Rolle said. "It's still unfortunate, but we've got to keep his spirits high and let him know everything is going to be OK. We're going to hold it down until he gets back.
"We know it's going to be a big loss for our defense. But we're going to do the best we can to keep things moving and try to get back on a positive track."