Ernesto Valverde's side have been on an excellent run of late, and their impressive draw against runaway leaders Barcelona last weekend left them sixth in the La Liga table, just two places and two points off a top-four spot.
Celta Vigo are struggling at the other end of the table and are a point adrift of safety in 18th.
But their home form has been key in keeping alive their hopes of survival - they have only lost twice at Balaidos in the league this season - and Parejo believes it is a test his side need to overcome, particularly with their Champions League last-16 tie against Paris St Germain coming up next week.
He told a press conference: "It will be crucial to get the three points in Vigo if we are to achieve the objectives we have set out for the season.
"We are only thinking about the game on Saturday and are taking things day by day, even though we know there will be a lot of focus on that match against PSG, but we have to keep our focus.
"We have a good dynamic, a good feeling in the squad and we have been winning away from home, something that we did not do at first.
"The coach made the decision to change our system and I think the 4-3-3 is the best system. It's going very well because we are now able to control the game."
Parejo is set to return from injury at Celta after three weeks out with the broken hand he suffered in the Copa del Rey meeting with Real Madrid at the Bernabeu.
But, having undergone surgery, the 23-year-old is raring to go.
"It's been progressing well and everything, but always after an intervention there is minimal risk. But I am happy to be back," he said.
Celta have only won one of their last nine league matches and their performance in last weekend's 1-0 defeat to Osasuna was described by coach Paco Herrera as their worst of the season.
Defender Carlos Bellvis is hoping for better on Saturday, saying: "The three points against Valencia are very important to close on the teams who are above the drop zone. For us it's a vital game, because the margin for error we have is now much less."