Defensive end Cameron Jordan (knee) is also listed as questionable on the injury report, but he should play after practicing on a limited basis all week.
Snead labeled himself as a game-time decision after practicing on a limited basis Saturday. But the Saints apparently wanted to give him at least one more week of rest and recovery after he said he stretched ligaments in his toe last week against the New York Giants.
"I don't feel 100 percent, but throughout the week my foot's been getting better and we've been treating it really well," Snead said Saturday. "I just want to be healthy for the year, to be honest. I don't want it to linger, I want it to be gone.
"It's Monday Night Football, the atmosphere's gonna be great. It's gonna be frustrating if I can't go. If I can go, I'll make the most of it. ... If I can't, I'll be on the sideline cheering my butt off."
With Snead out, it provides a great opportunity for rookie receiver Michael Thomas, who had an outstanding training camp and preseason for the Saints after being drafted in the second round out of Ohio State.
Thomas (6-foot-3, 212 pounds) has 10 catches for 114 yards on 11 targets this season.
"When your number's called, you have to respond," Thomas said of the possibility that Snead might not play. "I still [have] a lot of room for improvement. I'm heading in the right direction, but I'm not nearly close to how good I can be, and I just have to keep developing."
Armstead could either be replaced by left guard Andrus Peat or veteran backups Senio Kelemete or Tony Hills. Peat has struggled through some ups and downs since being drafted in the first round in 2015, but left tackle is his most natural position.
"I played there my whole life, high school and college," Peat said Saturday, although he stressed, "I'm just ready to play wherever."
The Saints will be at a huge disadvantage in the secondary since they are now without their top four cornerbacks from the start of training camp -- and they're facing Falcons wide receiver Julio Jones, among others.
But they have gotten used to the next-man-up mentality at that position. Last week, recently signed veteran Sterling Moore and undrafted rookies De'Vante Harris and Ken Crawley did a terrific job of holding tight against Odell Beckham Jr. and the Giants, holding them to zero offensive touchdowns in a 16-13 loss.