The Saints travel to the Stadium of Light on Wednesday for their last-16 clash looking to extend their unbeaten run in all competitions to 10 matches.
A stalemate at the Britannia Stadium took Mauricio Pochettino's men into sixth in the Premier League standings, with 19 points from their opening 10 fixtures.
Although Fonte and the rest of the players involved in Southampton's 1-1 draw at Stoke on Saturday are set to be rested, the defender remains confident the squad has strength in depth.
"It will be very hard but a few players want to show the manager he can count on them so we'll have a strong team," he told the Southern Daily Echo.
"There are a lot of other players that deserve to play. We have a very good squad with players that need minutes and game time.
"If we win this one we're at home in the quarter-finals, so hopefully we can win so we can welcome Chelsea, which would be a great game."
Despite Pochettino set to make a raft of changes, Saints will still be able to field several internationals with Tadanari Lee, Steve Davies and Gaston Ramirez all set for a recall, while Maya Yoshida is expected to renew his Capital One Cup partnership with Jos Hooiveld.
Sunderland have lost seven of their eight previous outings against Premier League opposition, but did pick up a home victory in the Tyne-Wear derby last month following the arrival of Gus Poyet as manager.