Mascherano returns for Liverpool opener
Mascherano missed Saturday's 4-0 Premier League rout of Burnley with a pelvic injury sustained on international duty with Argentina.
"He went to South America with a slight problem and played," Liverpool manager Rafa Benitez told reporters on Monday.
"Afterwards he needed to check it out so we decided he should do it over there and miss our game at the weekend. It was not serious, only a minor thing, and he will be fit to play in the Champions League."
Benitez has been more successful in Europe than in the Premier League since arriving at Anfield in 2004.
Liverpool beat AC Milan to win the tournament in 2005 before losing to the same team in the final two years later.
Benitez though is still looking to lift the Premier League trophy for the first time, having come close last season when his team finished second behind Manchester United.
"We have to know they are two massive competitions so both are important," said the Spaniard. "If you are asking what is more important, both are more or less at the same level but our fans would like to win the Premier League.
"We would be really pleased if we could do that but I don't think anyone would not want to win the Champions League so we have to approach the competition thinking that it is massive, a big trophy, and we will try to win it from the first day."
Hungarian champions Debrecen warmed up for their trip to Anfield with a 1-0 victory over Paks on Friday that took them to fifth in the table.
The Champions League new boys will be without midfielder Jozsef Varga and winger Tibor Dombi through suspension while forward Gergely Rudolf is missing with a thigh problem.
Olympique Lyon and Fiorentina are the other teams in Group E.