It'll cost him some pace but that's gone anyway. Players as old as him tend to loose quite a bit of pace after a long layoff that doesn't return.
By the time he returns and regains match fitness and adjusts to having less pace he's not going to have a lot of milage left at a top club. He has MLS written all over him.
Injecting a Achilles takes the pain away breifly but massively increases rupture risk for the next week. I wonder if we injected it. I'd like to think we didn't. David haye had his Achilles injected a few days before his boxing match and that's why it ruptured that time.