Messi has not scored in La Liga since Barca's 2-0 victory at Almeria in September - a drought of eight games.
The 26-year-old has struggled with injuries this season, and only returned from a two-month lay-off with a hamstring problem earlier this month.
Messi has scored four goals in three Copa del Rey appearances - as well as providing four assists for team-mates - since the turn of the year, though, having appeared to have taken up a more withdrawn role in Gerardo Martino's side.
And Real Madrid coach Ancelotti feels these statistics prove that the four-time FIFA Ballon d'Or winner remains as crucial as ever to Barca's fortunes, regardless of where he lines up.
"I don't know if Messi has changed position," said the Italian.
"He has played well in recent matches and shown that he is fit and has recovered well from his injury.
"He has not scored many goals, but he is still, as he always has been, very important for Barcelona."
Real sit just one point adrift of Liga leaders Barca and Atletico Madrid, and have won nine games in a row in all competitions ahead of Tuesday's Copa del Rey quarter-final second leg against Espanyol.
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