United only managed a 2-2 draw with Porto at Old Trafford last night.
He said: “The performance could have been better.
“It’s an uphill task now but one we’re definitely capable of.
“At least we know what we have to now in Porto: we’ve got to go there and win.”
He added: “They’re one of the best teams we’ve played this season.
“Their front three are all very dangerous, powerful players.
“They like to play on the counter-attack, which we will have to be wary of when we go out there.
“We know exactly what we’re up against now – the challenge has been set and it’s up to us to be equal to it.
“A draw at home in the Champions League isn’t a great result, but it’s not a disaster. It’s still all to play for.”