The League One visitors more than held their own for much of the first leg at White Hart Lane on Wednesday until Andros Townsend edged the hosts ahead with a 74th-minute spot-kick.
The winger stepped up after referee Neil Swarbrick had penalised Jay McEveley for handball, but Pochettino knows the tie is far from over.
"It was a tough game," the Tottenham boss said. "Now we go to Sheffield with a victory, but it will be tough up there.
"The first half was difficult to create chances. It was a good test.
"It's important to understand what we're up against. We need to go to Sheffield to win."
A full house at Bramall Lane next week for the return fixture will certainly provide Spurs with plenty to think about.
However, Ryan Mason believes Tottenham have the class to cope with anything thrown at them.
He added: "It was a difficult game. They came with a gameplan and did well. We're happy to go there with a lead.
"We're confident going into the second leg. We kept a clean sheet tonight and we'll go there to win."
