Although both have had severe injuries at young ages, Delph showed far, far more than Gardner at a younger age. All Gardner has done is rip up a youth league and dine out on his "good prospect" tag for several years. In his handful of appearances for us he hasn't looked anything special, and that was before the injury.
Delph was tearing up League One + the Championship for 2 seasons before coming to Villa, playing week in week out. He was being linked with teams far better than us, hence why MON paid £6m for him!
If Gardner's rotten luck with injuries continues he'll have to retire. It took Delph around 18 months to come back fully from his injury, but he had more time on his side than Gardner. I just can't see any future for him here - he may be offered a new contract out of sympathy however.