I'll be honest...when I looked, I frankly did NOT expect to see that Perez's age 25 season was basically what I highlighted off personal opinion. His age 22 season was even better than that and close to what I also highlighted.
Basically my feeling is an in the end BB/9=4 is probably a 3.5ish ERA, anything higher is just a worsening amount of bad ERA wise, but a decent BB/9 of 3.5 or so...which may not happen but is possible could mean a 2.8-2.9ish ERA. Good chance based off other lefty's careers...that we may end up seeing all of the above for his career...LOL.
Bottom line though is, lefties that have good velo are not going to have their starting chance shelved for a while.