irule syntax error

You can use an else condition:

when HTTP_REQUEST {
    log local0. "http uri: [HTTP::uri]"    

    if { ( [string tolower [HTTP::uri]] starts_with "/solr/spruat_slave" ) or ( [string tolower [HTTP::uri]] starts_with "/sol" ) } {
        log local0. "matched [HTTP::uri]"           
        pool mhf_pool_172.16.250.72-slave
    } else {           
        pool mhf_pool_172.16.250.72-80
    }

}

Or you can simply apply the mhf_pool_172.16.250.72-80 pool to the virtual server configuration. In either case you should also apply OneConnect - a setting in the virtual server and in the associated HTTP profile.