Below/under par
Definition 1 of 2
Pro Tip 1/3Health Status
Often used to describe someone not feeling well or lacking energy. 
I can't join you for the run; I'm a bit under par today.
Pro Tip 2/3Subjective Assessment
The phrase can be subjective based on personal expectations or standards. 
Her presentation was under par, at least by her usual high standards.
Pro Tip 3/3Work Quality
Describes suboptimal performance in professional or academic work. 
His latest project was criticized for being under par.