Forward Antoine Griezmann has said he struggled to adapt to life back at Atletico Madrid after rejoining the LaLiga champions from Barcelona last summer.
The France international starred this week in Atletico's dramatic 3-1 win away at FC Porto to secure qualification for the Champions League round of 16, but both he and the team have been inconsistent this season.
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Atletico go into Sunday's derby with Real Madrid 10 points behind their city rivals in the LaLiga table.
"To be honest it was quite hard for me at first," Griezmann told ESPN. "It was a rhythm I wasn't used to, a new system for me. I had to take that and mix it with my own style of play.
"Finally I'm getting better, and I'm feeling better on the pitch. To get to the next round of the Champions League was very important for me."
Griezmann, 30, spent five years at Atletico -- becoming the fourth highest goal scorer in the club's history -- before a controversial move to Barcelona in 2019.
After two up-and-down seasons at Camp Nou he returned to the Wanda Metropolitano in August on a season-long loan, with Atletico having the obligation to make the deal permanent next summer.
His contribution in LaLiga so far has been inconsistent, with three goals in 12 appearances, but he played a key role in Atletico's progression from Champions League Group B, scoring four times including the opener in the must-win clash against Porto.
An Atletico win in Sunday's derby would take them to within seven points of leaders Real with a game in hand.
"At the end of the day there are three points at stake," Griezmann said. "I don't think it really matters how many points ahead they are, because it's a really long season, we saw that last year... we still had to fight for the title until the last game.
"I think we can take something from that. There's still a lot of the league left and a lot of games left to play."
Diego Simeone's team go into the match facing a defensive injury crisis, with four of seven defenders -- Stefan Savic, Jose Maria Gimenez, Kieran Trippier and Sime Vrsaljko -- all likely to be absent, while striker Luis Suarez is also a doubt.
Real, who are also waiting on the fitness of forward Karim Benzema, have won their last nine games in all competitions.
"Real Madrid have been one of the best in the world for many years," Griezmann added. "They have always been there. It's a derby for us, so it's very important for the fans and the club. You are playing for each club's pride. It will be a great game to watch."