The American was withdrawn at half-time during the 1-0 defeat to Stoke City at Goodison Park on Boxing Day and subsequently missed Sunday's loss to Newcastle United.
Howard suffered an injury to his left calf when going to kick the ball and boss Roberto Martinez has now confirmed that the 35-year-old will be out of action for between five and six weeks.
The Spaniard also revealed he would look to assess his options in terms of signing a back-up goalkeeper in the January transfer window, with Russell Griffiths and Jindrich Stanek the only cover for Joel Robles.
"Tim is going to be a huge miss, not just on the pitch but just being around the place, bringing the calmness and know-how that he does to the younger players," Martinez told a media conference.
"I would love to find a positive on him being injured but I'm struggling.
"[Signing a goalkeeper is] something we'll assess. We just found out about Tim and weren't sure how long he would be out for but we'll make a decision because we've only got two young goalkeepers [as back-up].
"We need to assess that over the next few days. "
Martinez was, though, coy on the issue of signing other players in the transfer window, despite a lengthy injury list that includes the likes of Phil Jagielka, John Stones (both ankle), Leon Osman (foot) and Steven Pienaar (knee).
He also dismissed speculation that full-back Seamus Coleman is a target for Manchester United.
Asked whether there had been contact from Old Trafford over the Irishman, he said: "No. For me, it's nothing. I haven't seen that and have no recollection of that.
"The last thing we want to do is to lose important players.
"Windows are opportunities to see if we can find anyone who fits in with the club, I don't see the window as one we need to perform well in.
"Solutions can be found internally but if we can take advantage of any opportunity, we'll look into it."
