Posh began slowly and were already trailing to Paul Anderson's opener when Tomlin was shown an 11th-minute red card for felling defender Greg Cunningham with an outstretched arm.
Ferguson said: "I'm not going to blame the sending-off of Lee Tomlin for our defeat. We cannot start games as poorly as that and expect to get anything.
"I don't think Lee intentionally used his elbow on an opponent, but if you raise an arm you take a risk and we will now lose him for three games.
"That is a big blow because he has been in good form. But Bristol started really brightly and it was the goals we conceded, rather than the red card, which cost us."
Grant McCann brought Peterborough level from the penalty spot but Sam Baldock scored twice either side of a Stephen McManus header, before Dwight Gayle's late consolation.
Ferguson added: "The two in the second half were soft and put us out of the game. We must now brush ourselves down quickly for a really big game against Barnsley."
- Sports & Recreation
- Lee Tomlin
- Greg Cunningham