He did not pray in congregation because he was afraid that it would delay healing of his feet

Praise be to Allah.

May Allaah bless you for your eagerness to pray in congregation. I ask Allaah to make us and you steadfast in obeying Him and to accept it from us and from you. 

Yes, sickness is one of the excuses for which one may stay away from prayer in congregation. 

It says in al-Mawsoo’ah al-Fiqhiyyah (27/187): 

This refers to sickness which makes it difficult to go to the mosque in order to pray in congregation. 

Ibn al-Mundhir said: I do not know of any difference of opinion among the scholars concerning the fact that a sick person may stay away from prayer in congregation because of his sickness. When the Prophet (peace and blessings of Allaah be upon him) was sick he stayed away from the mosque and said: “Tell Abu Bakr to lead the people in prayer.” End quote. 

The fuqaha’ (may Allaah have mercy on them) have stated that if the sick person is afraid that his recovery may be delayed, then he is excused and the duty of worship is reduced in his case, whether it is prayer, fasting, purification or anything else.  

See: al-Insaaf (2/305) and al-Mawsoo’ah al-Fiqhiyyah (14/258). 

What counts in such cases is the opinion of a trustworthy doctor, or knowledge of that due to experience. 

If a great deal of walking will delay recovery, then there is no sin on you if you do not attend prayers in congregation in the mosque. So long as you pray regularly, you will have the reward of praying in congregation in sha Allaah, because of the hadeeth of Abu Moosa al-Ash’ari (may Allaah be pleased with him), according to which the Prophet (peace and blessings of Allaah be upon him) said: “If a person falls sick or travels, there will be recorded for him the same reward as that for what he did when he was healthy or was at home.” Narrated by al-Bukhaari (2834). 

And Allaah knows best.