The 26-year-old was grateful to the Premier League champions for beating Shakhtar Donetsk, while adding that the Werkself are having a fine season so far
The Bundesliga outfit beat Real Sociedad 1-0 on Tuesday evening courtesy of a solitary Omer Toprak strike, while the Red Devils beat Shakhtar Donetsk by the same scoreline to help the German side leapfrog the Ukrainians in the Group A table.
"Our qualification is, in part, thanks to United's win but we didn't know that until we saw it at the end on the giant scoreboard," midfield ace Castro told reporters.
"It was a tough match, evidenced by the fact we were fighting for the right result until the end of added time. We came here to win and we achieved that."
"Who we draw in the next round doesn't matter to me. We are there, that's what counts. It's early yet but, yes, the season is going well for Leverkusen thus far."
The draw for the round of 16 takes place in Nyon, Switzerland on Monday.