The 50-year-old insists there is an "urgency" to bolster his squad but admits his main targets are unlikely to switch clubs during the winter window.
The Premier League champions currently sit seventh in the table, 11 points behind leaders Arsenal, and have crashed out of the FA Cup third round following Sunday's 2-1 defeat at home to Swansea City.
Moyes had previously urged caution ahead of the window and has reiterated that the club is unlikely to sign its priority targets this month.
“I said that I would try to, but it would probably be doubtful in January,” the United manager told reporters. "There’s an urgency that we would like to bring people in but are those players available in January? The players we would like to bring in probably aren’t available in January.
“It’s not because we wouldn’t like to do it, it’s because they aren’t available.”
United went behind via Wayne Routledge’s early opener but pulled level through Javier Hernandez.
With the score level in the second half, Fabio, four minutes after coming on as a substitute, was sent off for a challenge on Jose Canas, while Wilfried Bony’s header fired Michael Laudrup’s side into the fourth round.
Moyes admits referee Mike Dean made the correct decision with Fabio’s dismissal but claims his side put in a performance worthy of securing a replay.
“I thought we played well enough,” said the Scot. “I didn’t think we deserved to lose but we did. We put out a really strong team today with lots of international players. We should have taken more chances when the opportunities were there. We’re just lacking a bit of courage when we get in to the final third at times.
“We need to get back to winning ways. We have to make sure we find a solution to win. It doesn’t matter what style, we have to find a way of winning. It’s been a tough start, a tough period. I’m disappointed not to win more games and play better, but I’m sure it will change.”