Phillips returned to action for Leeds against Brentford on February 11 after missing the previous three matches due to suspension.
Bielsa believes that the break in competitive football for Phillips actually made his performance level drop, but he recovered it last weekend.
When asked to explain, the Leeds head coach said: "He was three matches out. He played the match with under-23s. Technically he was very good. The attitude he played with was excellent. He had problems in coordination, speed, agility. In the match against Brentford he had few opportunities to touch the ball and organise the play. Just from the minute 30 to 45 he was in contact, the rest of the match no. He had good recoveries when danger was in front of him.
"The player he faced, 14, that is a number eight, neither defensive nor offensive. Not offensive like the playmakers he is used to playing. It’s like most of the playmakers of our opponents. He had problems to face this player when he changed direction with the ball. Those two situations, few contact with the ball to build attacks and lack of agility to adapt in defence with change of opponent’s direction are linked with situation he was out three matches.
"He lost a level as is natural. You can recover some things just playing, even this description he had positive impact in the match against Brentford for us because he knows how to cover our players in defence, but he recovers his real level in the last match. He defended against attackers or playmakers like the 20.
"There he had more impact in defence and more efficiency. To know what I am saying is true, normally Kalvin makes double or triple of the movements of high intensity of our centre-backs, maximal speed. Against Brentford and he didn’t do double or triple, he did half of the centre-backs, but last match he came back to normal values.
"That means he didn’t play three matches, the situation made him worse. After he started playing the situation improved. Those conclusions are not my opinion. It’s a description of the images and Kalvin’s play in the pitch. Not an opinion. One description.
Important player for Leeds United
Phillips is one of the best midfielders in the Championship and is a key figure in the current Leeds team.
The 24-year-old bosses the midfield department, and the Whites do not look the same team without him in it.
The Englishman has made 29 appearances in the Championship for Leeds so far this season, scoring one goal and providing two assists in the process, according to WhoScored.
During the 2018-19 campaign, Phillips scored one goal and provided three assists in 44 Championship games for Bielsa's team, according to WhoScored.
Leeds are second in the Championship table at the moment and are aiming for automatic promotion to the Premier League.
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