Thiago Alves has great leg kicks, but his boxing is average at best. Even Karo was outboxing him and Karo's stand-up is still just improving. Diego's been working on his stand-up for just as long as Karo, and with consequently better results, so I wouldn't be surprised if Diego lands more punches than Thiago.
That said, Thiago's a massive WW, and I suspect once he starts landing leg kicks, Diego's going to try bringing the fight to the ground. Thiago's had problems with smaller, quicker WW fighters on the ground, and Diego is probably the fastest/most technical smaller welterweight in the division.
I think Diego wins a unanimous decision.