The Swansea City manager was speaking as his side prepared for their replay with Fulham at Craven Cottage this evening.
He told the Evening Post “We are a stronger side than we were when we lost to Havant
“We are stronger mentally, technically and tactically, but the one thing we can take from that game going into this one is that anything can happen in the FA Cup.
“Us losing to Havant won’t be the last upset in the cup, and it’s nice for us that we can wear the underdog tag this time.
“It gives you that little bit of extra feeling and hopefully it can help us against Fulham.
“They are in a good moment of form and their home record is fantastic, but we go there with hope and that’s a great compliment to us.
“But realistically, every time we take on a Premier League club we need a good performance, a bit of luck and a bit of magic of the cup to get through.
“They will look to dictate things and they have four strikers who can score out of nothing,” added Martinez.
“We have to go there and try to compete and, if we can do that, I will be happy.
“The fans will dream about United, but all I want is a performance which shows we are growing and will help us in our league campaign.
“We have still got options in the squad and we must look for solutions in the squad first.
“Joe was enjoying his football and he has had a massive impact. It’s unfortunate that we are going to miss him and Garry, two big players, but we have to deal with it as a squad.”