Cooper was withdrawn in the first half after falling awkwardly in the penalty area while Phillips was withdrawn with 10 minutes gone in the second half.
Bielsa was unable to give a prognosis on either of the players as Patrick Bamford scored a stoppage-time equaliser to rescue a point for Leeds in the 2-2 draw.
Bamford was making his return to first-team action after picking up an ankle injury against Newcastle United in September. Luke Ayling also made his return to the side after sustaining an injury in the same game.
"Ayling had a satisfactory game, Patrick scored a goalscorer's goal," reflected Bielsa.
"The sector of the pitch [Bamford] came on in, it was difficult to find the ball but finally he gave us the equaliser that for our expectations is very valuable, given how we got it.
"Before the game it wasn't a good result, after the first half it wasn't as well but when the game was finishing we need to value what we got.
"The players that are used to scoring normally they're in the right place at the right moment. What you value about the score is the opportunism of it."
FA CUP -Leeds United' s ball number for Monday night's third round draw.
Leeds had taken the lead at Elland Road on Sunday afternoon as Raphinha's inviting cross was converted by Tyler Roberts.
But Brentford hit back with two goals in seven second-half minutes as Shandon Baptiste and Sergi Canos both found the net to put the visitors ahead.
But Bamford popped up late on as he scrambled in at the back post from a corner kick. Bielsa was pleased to pick up the point but felt Leeds should have had all three.
He added: "To have drawn the game the way we did changes the flavour.
"But if you look at the game in its general context, we should have won it. We conceded the first part of the game, but without conceding any goals.
"From the initial 15 minutes onwards we had a good half an hour. We started the second half well and we had incomings and outgoings like Ayling and Phillips where we were a few moments without them.
"Kalvin went back on and came back off, that made us lose organisation but it shouldn't have been this way. In those 15 minutes in the second half we did receive the goals.
"But the final 15 minutes, with a lot more difficulties but a lot of perseverance to attack, I think we ended up getting a result that was fair on one side and insufficient if you look at the whole game.
"We dominated two thirds of the game and we duplicated the amount of dangerous situations we had, compared to the opponent."
In more positive news for the Whites, Bielsa revealed that Pascal Struijk is close to a full recovery from a hip issue.