Kubiak was taken to hospital after falling to his knees while leaving the field at half time of his side's match against the Indianapolis Colts last night, although general manager Rick Smith has eased concerns over the 52-year-old's health.
Phillips took over for the second half, but admitted that he was dogged by uncertainty regarding matters on the field and Kubiak's condition off it.
"There was a lot of unknown. Everything was unknown as to what was going on and what happened to Kub," Phillips told The AP.
"[Offensive coordinator] Rick Dennison obviously called the plays from the press box. We had to adjust as far as the head coach not being there. But it was a shock to everybody."
The Texans had been leading 21-3 before Kubiak's collapse but suffered a second-half meltdown as the Colts came back to win 27-24.