This may be technically demanding, but why not have the option for players to place themselves in a queue to join a game that has uneven odds.  For example, a game starts and team A and B both have 3 players, but A loses one player due to disconnect.  The game then searches the queue for a player to connect to the game with a ping not exceeding a certain ping threshold.  The character joins the game at the average level of all players rounded up, is given a certain amount of gold based on level, and earns a large favor bonus for joining.  There could also be additional perks for those helping out the team, such as greatly increased personal gold rate, access to unique items, increased experience gain, etc.  If it's a pantheon game, you could also have it only positively affect their rating.  While a team is down a player, another suggestion, is to increase the power of all units/towers/and remaining demigods, and increase gold/exp rate to compensate.  If team B also loses one player, the search in the queue ends, and all increases in power to the previously undermanned team ends.

An AI teammate is not a benefit to a team who has lost a player, it can be a monumental hindrance!