Reading appeared set for their first league victory over the European champions for 82 years when they led 2-1 with 20 minutes left of a thrilling match.
But England centre-half Gary Cahill put Chelsea level with a long-range effort before Torres scored from close in. Branislav Ivanovic added a fourth in injury time to put Chelsea on top of the Premier League after two games.
"We expected them to be tough and they made it very hard for us," Chelsea manager Roberto Di Matteo told Sky Sports. "In the second half we had to take a bit of a risk. We were good for a goal and I thought it was just a matter of time."
Chelsea dominated possession for the first 25 minutes at Stamford Bridge and Reading were forced into desperate defence to keep out a series of counter-attacks.
Juan Mata and Eden Hazard, the close-season signing from Lille, were linking up superbly and the Belgian earned his second penalty in two games after 18 minutes when Chris Gunter took his legs away in the area.
Lampard scored from the spot, just as he had early in Chelsea's opening 2-0 win at Wigan on Sunday.
The game was turned on its head, however, inside the next 10 minutes. First, Reading's Russian striker Pavel Pogrebnyak got ahead of John Terry to meet a fine cross by Gareth McCleary and send a glancing header into the top corner.
Danny Guthrie then smacked a free-kick through the Chelsea wall and goalkeeper Petr Cech could only spill the ball into his own net.
The Chelsea defence was shaken and a free-kick by Ian Harte almost led to a third Reading goal but Alex Pearce headed wide when unmarked.
"We started very brightly, played great stuff, but the goals knocked us back a bit," said Di Matteo.
Reading looked comfortable protecting their lead after the break, keeping a close eye on Torres whose industry up front was not matched by his shooting accuracy.
Oscar replaced fellow Brazilian midfielder Ramires after an hour but Chelsea continued to struggle for rhythm in a match rearranged because the home side face Europa League winners Atletico Madrid in the European Super Cup in Monaco next week.
But Cahill put Chelsea back on level terms when he thumped a 25-yard shot through Reading goalkeeper Adam Federici before Torres scored from an Ashley Cole pass in the 81st minute.
Replays showed the Spain striker may have been offside and Reading manager Brian McDermott said he was "gutted" by the linesman's "mistake".
Reading threw everything into attack, including goalkeeper Federici, in the dying seconds and Chelsea broke away to leave defender Ivanovic with a simple task to score his second goal in two matches.
"I certainly don't feel we deserved to lose the game," McDermott told the BBC. "We've come to the European champions and put on a real performance and it shows we deserve to be here."
