Favre suffered a concussion in the second quarter on Monday night in the Vikings' loss to Chicago when he was slammed to the turf at TCF Bank Stadium on the University of Minnesota campus.
Favre, 41, hadn't even been expected to play in that game because he was declared "out" on the injury report two days earlier due to an injury to his shoulder and throwing arm. That injury is what had ended Favre's NFL record streak of consecutive starts the previous Monday against the New York Giants.
Rookie Joe Webb had prepared to start against Chicago - he did come in after Favre went out - and Webb is again preparing this week as if he's going to be the starter on Sunday night against Philadelphia.
It appears there is a real chance Favre won't start for the second time in three games because of the concussion. Even if he passes the baseline portion of the test, he is still going to need to get through the physical testing.
He can continue to be tested through Saturday before a final decision has to be made. Playing against an Eagles defense that leads the NFL in interceptions and gives multiple different looks at the line of scrimmage, Webb could be in for a rough day.
Webb has seen action in the past two games at quarterback after not playing all season. He has completed 17 of 31 passes for 137 yards with no touchdowns, two interceptions and a 39.3 passer rating.
When asked what he's looking for from Webb, interim coach Leslie Frazier said: "It's just to see if his command of the offense is a little bit better. Just to see if his accuracy is even better. Just to see if the game has kind of slowed down for him and he's able to function better than he did in his first start. And if you can see some improvements in those areas, it gives you some hope for the future. And I think he will make improvement. He's a smart guy. He's shown a command in practice. We've just got to see how he does against a very good defense on Sunday night football."