Penske driver Hornish has had a tough time since moving from IndyCar to NASCAR in 2008 - finishing only 35th and 28th in the Sprint Cup standings in the past two years.
With Patrick entering an exploratory programme of ARCA and NASCAR Nationwide races this year as she ponders a similar move, Hornish said she should expect to find much greater strength in depth in stock car racing.
"I can only say that I know how many races I won over in the IRL and championships," said Hornish. "You can look and see the difficulty that the transition has been for me. I hope that she does well with it and that things go well. For me going through it, I know it's a very, very difficult change to make.
"I think that there are people as talented over here [in NASCAR]. But instead of having five guys that are that talented, you have 30 guys that are that talented [in NASCAR]. You say, 'Oh, there's double the cars.' Well, okay... there's double the cars but there are twice as many teams.
"I just look at the competition: this is the fastest car in the field in qualifying, this is the last car. How close is it? How many guys are in this amount of time and how hard is it to win?
"If you're a sixth place car over there [in IndyCar] and you are on the same talent level coming over here, you're going to be 30th."
Hornish also thinks Patrick is being unwise to race in both NASCAR and IndyCar this season.
"I talked to Dario Franchitti about it when he came over and he told me, 'Man, I can't believe how difficult it would be to juggle both of them back and forth,'" said Hornish.
"It's not the way that I would do it. I would come over [full-time], but since she doesn't know if she wants to do this full-time, I think she's doing the right thing - learning.
"Don't jump into Cup full-time next year. Give yourself a year in Nationwide just to learn. Give yourself an opportunity to be able to race closer up to the front instead of spending a full year of running between 20th and 30th."
Patrick will make her stock car debut in the ARCA race at Daytona this weekend, but has not yet decided whether she will take part in the Nationwide Series event at the track a week later.
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