What might cause numbness in the foot after deep tissue massage?

Numbness: Numbness is caused by the nerve not functioning properly. Excessive pressure can cause the nerve to fail to send signals until it has recovered.

Deep tissue massage: Can cause a refractory numbness called neuropraxia. This can happen when the nerve is irritated by pressure or combination of tissues entrapping the nerve. This is usually short-term and will eventually go away. If it doesn't, see your podiatrist, who can isolate the nerve with an injection of Lidocaine to differentiate inflammation vs. Neuropathy.

Nerve: And vascular compression. I would have the therapist ease up.