Sunderland - Dutch striker Vincent Janssen believes South Korean team-mate Son Heung-Min is demonstrating his growing importance to Mauricio Pochettino's Tottenham Hotspur squad.
Son helped save Spurs from an FA Cup embarrassment at the hands of Wycombe Wanderers on Saturday and is in contention to retain his starting spot for the Premier League visit to bottom club Sunderland.
The 24-year-old South Korean international struck twice as Pochettino's side fought back after trailing by two goals to the League Two side, eventually scoring the added time winner that booked a place in the fifth round.
The victory brought a huge sense of relief to White Hart Lane, but the team's underwhelming display raised questions about Pochettino's claim his squad is strong enough to sustain a Premier League title challenge.
The manager had made nine changes to the side that started in the draw with Manchester City the previous weekend, but Janssen - who appeared as a second half substitute - believes Son should be above criticism.
"If you see what Sonny has done all season, it's incredible," Janssen told the club's official website.
"He's done well in every game and when he is on the pitch he shows what he can do.
"He is working very hard as well and you can see what kind of qualities he has. He's a very, very good player, he helped us today and he's helped us all season.
"He was very important and he got us through to the next round."
Son led the attack in the absence of Harry Kane who was ruled out after picking up a knock against City.
The England striker is expected to be fit enough to make the trip to the Stadium of Light which could mean Son drops to the bench or operates in a deeper role.
The draw at City ended a run of six straight league victories that included the defeat of leaders Chelsea and Pochettino will be well aware his side have to piece together another winning streak if they are to close the gap on Antonio Conte's side.
Spurs are nine points behind the leaders, but are about to enter a run of 12 league games that throws up just one meeting - away at Liverpool - with one of the current top six.
Kieran Trippier picked up a hip injury against Wycombe, and joins fellow defenders Toby Alderweireld, Jan Vertonghen and Danny Rose on the injury list.
Sunderland are rock bottom after failing to win in their last seven games as injuries begin to mount for David Moyes' squad.
Eight players are currently unavailable in what has become the Wearside club's fifth consecutive relegation fight.
Swedish midfielder Seb Larsson revealed: "Some days we walk out to train and we've only had eight or nine fit lads from the first team, which I don't think I've experienced before.
"It's frustrating for the manager not to have all his squad available, but you can't dwell on it."
Senegalese defender Papy Djilobodji starts a four-match suspension for a violent conduct charge, but Lamine Kone and Didier Ndong after back from Africa Cup of Nations duty.
Joleon Lescott returns for the first time after the free agent arrived at the Stadium of Light on a short-term contract until the end of the season.
The former England centre-back is likely to be on the bench, having not played since early October during a brief spell at Greek club AEK Athens.
Moyes remains confident of bringing in Bordeaux's 26-year-old German full-back Diego Contento to replace Dutch defender Patrick van Aanholt, who has joined Crystal Palace.