LEFT JOIN with WHERE in the field of the first table OR field of the second table

SELECT COALESCE(tb2.numero_novo, tb1.numero_original) as numero
FROM
tb1
LEFT JOIN tb2 ON tb2.id = tb1.id
WHERE 
tb1.numero_original = <PARAM> OR tb2.numero_novo = <PARAM>

The above query generates a full table scan. How to optimize this query?

EXPLAIN:

+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys  | key            | key_len | ref  | rows     | Extra                                              |
+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+
|  1 | SIMPLE      | tb1   | index | numero_original| numero_original| 5       | NULL | 11683843 | Using index                                        |
|  1 | SIMPLE      | tb2   | ALL   | PRIMARY        | NULL           | NULL    | NULL |        2 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+----------------+----------------+---------+------+----------+----------------------------------------------------+

The problem I need to solve is:
tb1 is a table that contains the numero_original field. I want to create the tb2 table to store a new value for this field, (which I called numero_novo ). I did this because I can not change the value of the numero_original field, and I would not like to create the numero_novo field in the tb1 table, since it would be NULL in more than 95% of the 11 million records table. >

EDIT:

As they talked about indexes, I'm putting more information:

• tb1 has as its primary key the id field;
• The numero_original field of the tb1 table has index;
• tb2 has as its primary key the fields tb1_id and numero_novo ;
• The tb1 table has 11mi + records;
• The newly created tb2 table has 2 test records.

.

mysql> show indexes from tb1;
+-------+------------+-------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name          | Seq_in_index | Column_name     | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tb1   |          0 | PRIMARY           |            1 | id              | A         |    11066164 | NULL     | NULL   |      | BTREE      |         |               |
| tb1   |          1 | numero_original   |            1 | numero_original | A         |     5533082 | NULL     | NULL   | YES  | BTREE      |         |               |
+-------+------------+-------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

mysql> show indexes from tb2;
+-------+------------+------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name   | Seq_in_index | Column_name    | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| tb2   |          0 | PRIMARY    |            1 | tb1_id         | A         |           2 | NULL     | NULL   |      | BTREE      |         |               |
| tb2   |          0 | PRIMARY    |            2 | numero_novo    | A         |           2 | NULL     | NULL   |      | BTREE      |         |               |
+-------+------------+------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

EXPLAIN selecting with WHERE only in the first table ( tb1 ):

+----+-------------+-------+------+----------------+----------------+---------+-------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys  | key            | key_len | ref   | rows | Extra                                              |
+----+-------------+-------+------+----------------+----------------+---------+-------+------+----------------------------------------------------+
|  1 | SIMPLE      | tb1   | ref  | numero_original| numero_original| 5       | const |    4 | Using index                                        |
|  1 | SIMPLE      | tb2   | ALL  | PRIMARY        | NULL           | NULL    | NULL  |    2 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+----------------+----------------+---------+-------+------+----------------------------------------------------+

EXPLAIN selecting with WHERE only in the second table ( tb2 ):

+----+-------------+-------+--------+---------------+---------+---------+-------------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref                     | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+------+-------------+
|  1 | SIMPLE      | tb2   | ALL    | PRIMARY       | NULL    | NULL    | NULL                    |    2 | Using where |
|  1 | SIMPLE      | tb1   | eq_ref | PRIMARY       | PRIMARY | 4       | sistema.tb2.tb1_id      |    1 | NULL        |
+----+-------------+-------+--------+---------------+---------+---------+-------------------------+------+-------------+