Professional Footballers' Association chief executive Gordon Taylor claimed Pennant was at a "crossroads in his career".
He said the prison term may be a turning point in Pennant's life, but admitted a custodial sentence had looked likely.
He said: "It's not entirely surprising, bearing in mind being a football player does not mean you are above the law, and the previous history gave an indication that there could be a custodial sentence.
"It's a big disappointment for the lad and it's a serious situation and a real crisis in his career. He was a very promising youngster, Arsenal paid a lot of money for him and it's not quite worked out.
"Birmingham was a chance for him but this has interrupted that now so this is really a crossroads in his career and I hope he has got the strength to come out of this and be better for it. The PFA will be trying to help him do that along with the necessary support of his family and friends."
Roger Vincent, of Birmingham- based Royal Society for the Prevention of Accidents, said: "Whether you are a well known footballer or not, this can lead to you going to prison.
"But it is worrying that people are prepared to get behind a wheel after they have had a drink."
Mary Williams, chief executive of road safety charity Brake, said: "We welcome the fact that the law has finally shown the red card to a drinkdrive footballer, but three months is woefully inadequate considering Pennant was already banned from driving."