They should've been able to right away. The problem is that they weren't practicing for it at all this past week.
On defense, they were working on defending Wallace deep (Slater lined up 2 yards past the LoS in practice this past week to simulate Wallace's speed) and containing Roethlisberger in the pocket. Well, Wallace averaged 10 yards per reception with a long of 16, and Ben didn't accomplish a whole lot outside of the pocket. He was perfectly fine with sitting back and getting the Pats inside and underneath rather than outside and deep.
Here's a quote from Arrington:
There was no bomb to Wallace for 40 yards on a fade route. There were just a lot of slants and such to him. The team was not prepared.
On offense, the Pats were obviously preparing for zone blitzes. It's the Steelers. That's what you prepare for. Well, they abandoned that. When the Pats sent 5 guys out on routes on 3rd and long, the Steelers played 2-deep, man underneath while rushing 4. That's not what you get from the Steelers, so the Patriots weren't expecting it. Unfortunately, that's what they got.
I said earlier this week that Pittsburgh's only chance was to completely switch things up. Well, they did.