The minutes weren't that different between the seasons. He actually made more appearances in 08-09 that the season before though.
United signed him on a 2 year loan in 2007 with the option to sign him for £25m in 2009 when his loan ended. United offered up the £25m in 2009 as agreed but Carlos and his owner chose the money on offer form City instead. Not sure what else Unted could have done.
Tevez wasn't a world class player for United and I suspect he would have never reached the level he did at City playing 2nd fiddle to Rooney had he stayed at United. He wasn't the most professional off the pitch to say the least and by the summer of 2009 I think Fergie wanted him but wasn't fussed he left either as he was a bit of a drama queen.
There number of minutes were quite different, not massive, but not a little difference.
Carlos asked to be signed after his first season and accelerate the process and Fergie told him what you are saying, to wait to the end of the loan while adding that he would go for Berbatov, not the best of moves, maybe another less high profile forward wouldn't be that much of a bad sign for Carlos.
Later Berbatov arrived and played more minutes than him (not a lot much, but more anyway), so he felt what I've said before, that they were putting lots of money (at the time) for a new high profile player instead of first assure him his first team status. And in the meantime, let's not be naif, try to keep the original ammount or even lower it if Carlos does not participate more like it should have been after what he did in his first season and without Cron in the team. To add things to the mix, Fergie neither gave full trsut to Berba in some ocassions, to turn things even greyer.
Like you've said Fergie didn't wanted him that badly, it's his prerrogative, even if wanting him, but do not act surpirse if thinsg end badly with a fella of the charcater of Carlos, he is not "yEs Boss" Neville type.
Fergie always tend to get things like he liked it and that situation infuriated him, more when Tevez ended in fecking City, that's why I always felt he didn't read well with whom he was dealing, for the good and bad, because like you've said, even buying him would not assure a smooth relationship, Carlos could have end badly in Man Utd too, but trsut me he really loved his Man Utd days even today, maybe that's why he took it so badly.
Yet all in all it was a turning point, for Man Utd and for City that if Fergie played his cards more wisely (and listened more to his lockeroom, specially Wazza), that if Carlos wasn't as stubborn as he is and didn't let Kia to be so important, things could have ended etter.
I trully think that without Cron in the team and with the relationship Rooney had with him, they would have done great, no matter if Wayne would have kept the number one spot. Tevez whole Robin approach, second fiddle was in order to gain that contract, the Carlos from Boca, Corinthians and post Man Utd teams, would have been there. I think that even this last aspect made it worse, that he let his big ego aside (maybe the only tiem in his whole carreer) for his first season with Man Utd and he never felt comfortable, nor THAT wanted, while not receiving the best signs in his second one. Shyte happens