The Whites midfielder was forced off the field in the latter stages of the first half after he had tried to carry on after picking up a knock.
The 24-year-old's return after a three-game suspension has coincided with Leeds' upturn in form as they made it seven points from the last nine on offer with victory over Reading.
"I think it’s a kick. I don’t know if it’s serious or not," said Bielsa.
Gjanni Alioski came on in Phillips's place with Ben White moving into defensive midfield while Luke Ayling shifted into centre back with Stuart Dallas switching to right back and Alioski playing at left back.
Bielsa was happy with the changes and felt the work of Pablo Hernandez and Mateusz Klich helped to off set the loss of Phillips.
"White helped the team with a solid difference, but Kalvin managed the ball in a different way to White," he added.
"The help of Pablo and Klich to build the play out was enough to replace the distribution of Kalvin."
Bielsa felt that his side dominated the game as they opened up a five-point gap between themselves and third-placed Fulham.
Nottingham Forest and Brentford also failed to win this weekend as United strengthened their grip on an automatic promotion place.
Despite all the hosts' hard work, Reading captain Liam Moore almost grabbed an unlikely equaliser in the final minute.
"It was a similar match as the matches we are used to playing," Bielsa added.
"Generally, we create double or triple the chances of the opponent. We have more possession than the opponent, today this was the situation.
"They have some attackers that are not easy to control. Also, there was a situation in the last minute for them, we should have resolved.
"The number six became a striker which created confusion we could have solved from the bench."